Some transactions take more space than others, depending on a few factors like if its a native segwit transaction, how many UTXOs had to be used, how many outputs there are. You could have a block that only fits 100 transactions, and another that fits 3000, just due to the size of the transactions; so miners price things per byte (v-byte, see segwit) to get maximum fees per block. 1 transaction that takes 250kb but only pays .000,01B isn't worth 100 transactions that take 2.kB and pay .000,001B each.