Duolingo 400 Quest Explorer

Today I was able to achieve a new achievement called the quest explorer and earning 400 XP points.

Currently I'm getting a lot of numbering factors between things, minutes, months and even floors.

The trick is that some categories change the pronouncing of the number. For example the kanji 四
Four things is pronounced yottsu 四つ
However when it comes to other categories like months (四月). It's pronounced shigatsu しがつ

It doesn't only happen with four but also two or ni (に 二). When you talk about people, 'ni' becomes 'fu' when it comes to people, futari (二人).

Hopefully I can get more professional explanation on how this happen and if there is any technique to understand it.