Races created via the race builder use the same creation code as the game does. However, all horses take an equal performance hit. This is because league and constructed races can be expected to be of an unrealistically high quality. Other than this I can only think of two reasons for unpredictable form:

Pace. League races are much higher quality than any game races you are likely to encounter. The faster average pace will likely play to certain horses strengths and hinder others.

Seasonal form peaks. When exporting a horse you basically take a snap shot of the horse at that point in time. However, some horses are disposed to over performing or under performing at certain times of the year and the race builder races do not have a date attached so any peaks or troughs in the horses form that are related to seasonal factors would not come into play. I doubt this is a big factor but it may be an option is added in the future to attach a date to a constructed race (or at least a league race).

They may be other factors but right now I can't think of any.

ya good advice i dont know why any1 would spend so much time using CK if they dont intend to enter the lge
yes its great to export your horses at the end of the year and trial them for breeding purposes like SC does but its basically a lge tool

If you enter the lge with the horses that have served you well in the game and not horses that excel in CK you will get a rude awakening
in my first year of the lge i was a bit arrogant and thought my horses were superstars i never trialed or went near CK i just used in game
stars and they bombed

ARAPHO the best tool at your disposal are your home gallops CK is not going to do much for you my friend
i would advise a few home gallop trials in January i usually do 3 or 4 before i race them when i was playing for fun

Speaking as a lge trainer that has run 10s of thousands of trials the cream always rises to the top
the most consistent horses will show there true ability in 7 or 8 trials out of 10
you get an inconsistent horse run a fast race in 1 or 2 trials out of 10 but they bomb in the other 8 or 9 trials
the CK is so valuable to find the most consistent horses and they end up being the quickest trialers or very close to it

an example in 5f trials i ran a few weeks back

horse A ran 57.55 in 1 trail a freakish time but never repeated it and was down the field in the other 9 trials
horse B has a best time of 57.77 but averages 57.8 in most trials and won 4 of them 2nd in another 4

i know which 1 i am taking to the lge the consistent horse always
i always dump any 2yo that has under 75% consistency BUT i have bred consistency in to all of my horses now

Heres what i look for when breeding and training for LGE
this is a genuine horse no altered stats as you can see some stats are maxed and some are low some rock bottom
this explains what i look for when i first study unraced 2yo and 3yo at start of the season

Mark wrote:

Races created via the race builder use the same creation code as the game does. However, all horses take an equal performance hit. This is because league and constructed races can be expected to be of an unrealistically high quality. Other than this I can only think of two reasons for unpredictable form:

Pace. League races are much higher quality than any game races you are likely to encounter. The faster average pace will likely play to certain horses strengths and hinder others.

Seasonal form peaks. When exporting a horse you basically take a snap shot of the horse at that point in time. However, some horses are disposed to over performing or under performing at certain times of the year and the race builder races do not have a date attached so any peaks or troughs in the horses form that are related to seasonal factors would not come into play. I doubt this is a big factor but it may be an option is added in the future to attach a date to a constructed race (or at least a league race).

They may be other factors but right now I can't think of any.

JEEZ! thank you Mark for your kind and 'pertinant answer' to my question of why the difference ? I am much obliged.(did'nt expect an answer from the 'Boss')
yours with thanks again
ARAPHO