Why do first bitcoin transactions receive most confirmations?
Once a transaction has one confirmation, it gets a confirmation whenever the network creates a new block (roughly every 10 minutes). If one of your transactions gets a confirmation, the rest will also get a confirmation if they have at least one confirmation already. So the difference in non-zero confirmations between your transactions will always remain the same (except in very rare cases when the block chain gets reorganized).

Going from 0 confirmations to 1 works differently.

https://en.bitcoin.it/wiki/Transactions
https://en.bitcoin.it/wiki/Blocks
https://en.bitcoin.it/wiki/Block_chain