If I may make a suggestion as to why...
The Bot teams use a default line-up and (I think) have their substitution pattern set to Let Coach Decide. The Bot coach always sends out what the ME thinks is the best player for each position whereas us human players may pick players according to training needs and they might not be the best player for a given match-up.
but maybe there is something unexpected going on.
Ahhhh....the wonderful feeling of emergent behaviour!
Or it might just be random.