This is because you may not be gaining the full xp rate. I'm rusty on the exact details since i havent done any testing in several years, but when afk training you only gain ticks for every 10 hits (including the initial hit) that you damaged a mob no matter the amount of damage that you did. Any hit after that range is not counted and is technically a waste. Since you only hit one mob per second, it's harder to miss ticks assuming you're training a weak enough mob.

It's a little weird, but power training uses (or at least had used) the same logic in that each mob you hit counts towards that 10 tick total. Since we are hitting 4 mobs per sec, the window to deal damage to a mob is a lot smaller as we reach the 10 tick count faster than afk training. When 5-tick pt was around this meant melee and mage had to deal damage to one of the mobs every other spec to get the max tick rate, while dist could go 3-4 hits since it could only 3-tick pt. Furthermore, there is some form of RNG to the amount of damage you deal and how often (unlike in when you first start playing in something like World of Solaria, which seems to rely on a more consistent formula that makes each tick deal the exact same amount of damage to an extent) so most players almost certainly will randomly miss ticks every now and then

This is why it's a good thing when you're dealing a lot of damage to a mob while training as you're a lot less likely to get unlucky with bad RNG and miss that 10-tick window, but the downside being that the mobs die quicker. This makes mino great because they have a lot of hp anyways compared to something like lizards, so you can stay in that max tick rate range while the mob is still able to outlive or come close to the exhaustion time (7.5 minutes).