The point is the number of transactions in a block could vary, and not require additional hashpower.

Obviously it's limited now by the blocksize (let's not go there). But the point is the "per transaction" figure is based on the currently imposed limits.

Okay, but this has nothing to do with the fact that miners do one job, and this job is processing transactions. They don't have another job that consists in "securing the network", in addition to processing transactions. Therefore the entirety of the energy spent by miners is spent processing transactions.