I know this happens in every family with more than one kid. "His half is biggerrrr!" "No fair! It's my turn to play with the toy!" "Why did you give her a bigger slice than meeee??" It just can't always be equal for siblings.
Fast forward to college.
We got the same cheap bike, cruisers (read: one gear). Mine has been fine all year. Baby A's? Looks like it lost more than one fight.
One of the two springs under the seat fell off. The end of one of her handlebars cracked and came off, so she ripped the end of the other one off to make it look slightly less ghetto at first glance. (Glance again, and the ghettoness is evident). She's been missing a bolt on one of her fenders forever, and that's why her fender caught her tire, leading to the epic bike accident I was not there to prevent. Her fender is now a crunched piece of metal under her bed. I think she's looking for a way to recycle it.
I have apologized for taking the good bike. See, before we moved to school, we were in the garage at Dad's...
Baby A: "Which one do you want?"
Baby B: "I don't really care. They are the same."
Baby A: "OK, well this one will be mine, and that one will be yours."
(I realize the bike disparity was beyond my control. Doesn't mean I can't feel guilty about it, K?)
No matter how the "same" things are, you just can't anticipate how different they will end up being. Like how that ONE EXTRA CRUMB looks fine to you, but is clearly evidence of your favoritism towards the other one. I'm sure I too had that irrational, small person mindset at some point, but I can't illuminate the rationale behind it for you. Sorry. Just take a deep breath, and remember "They will grow out of it."