Differences In Clones Due to Instructors/Field Experience/Etc.
Let's start with instructors- for this i'm going to use 'Republic Commando, Hard Contact' as an example.
To start off with, there are multiple times the clones in the story discuss their purpose. One of the Main Characters, A republic commando by the name of Darman, Is waiting after the battle of geonosis for his squad, which turns out to have all been killed in action. Forced to take the last transport back, he has a short winded discussion with a Infantry Soldier, where the Standard infantryman, bred specifically for fighting large land battles envies his upgraded weapons.
Darman stops to think later
"They don't train like us. They don't fight like us. They don't even
talk like us."
So, this can lead us to believe that clones are more or less trained specifically for their position.
Next up we have field Promotions, or just promotions in general. In some of the visual dictionaries, It is stated that the Kaminoans looked for clones that exhibited certain abilities, and they were taken in for specific units, such as starfighter pilots, armor specialists and bomb squad units.
Individual training- Again, going back to Republic Commando. It goes in the story that the special units were trained by well known bounty hunters and other mandalorians. Some of the ARC Nulls,(The Best of the Best, clones so dangerous, they were essentially raw Jango. They pretty much didn't answer to anyone but themselves, and in a comic I can't remember the name of, they take one out of actual cryostasis during an invasion of Kamino. Instead of allowing the Separatists to take the cloning facility, he has dying orders from jango to blow the place to hell instead of ever allowing it to be captured. In the end he doesn't, but this just goes to show how genetically independent clones were due to how they were bred and who they were trained by.
Last but not least, We can take example from SWTCW S7. The Bad Batch, These guys are black ops. troops. Independant, only called in if there is an extremely dirty job. Because of their genetic mutations, they are much more capable then standard troops, and can do pretty much any job.
Basically it boils down to genetics, and how they were bred for the position. Some of it may be to do with outside fores such as specific trainers, or how well they did in action, but a lot of it is based on how they were bred.
Just remember- It's nothing like the simulations.