Do the English and Japanese "season/series" start and stop episodes differ sometimes?
Yes. This happens a good amount to be honest.
Original series, Kanto to Johto, in Japan they only had 3 seasons while in English, there was 5 seasons.
In Hoenn and Battle Frontier, Japan had 2 seasons and English had 4.
In Sinnoh, Japan had 1 and English had 4.
Unova, Japan had and English had the same, but started at different times.
Kalos, Japan had 2 and English had 3.
And so far for Alola, Japan has had 1 and English will have 2 when it starts back,
So Japan has had 13 seasons and English has had 21 and about to be 22.
This is mainly due to the fact that Dub usually stops seasons at the end of the year, so they are limited while in Japan, it keeps going. The only time English doesn't stop showing is when there is only a hand full of episodes left. I think both Kalos and Sinnoh kept going after the end of the year because there was only 5 to 15 episodes left.