Bug portals, mobs and villagers

There is a bug that I don't know how to explain it, what happens is that not all the mobs pass from the other side of the portal, as I show in the picture, I appear many mobs but only 1 appears in the nether, there are times that not even one is on the other side, the same happens with the villagers.
In the picture it is explained better

The portals are well synchronized