Why Google is decreasing no of index pages..?

[ankitasharma123]

Hello friends,

Why Google is decreasing no of index pages..?

[Fli]

Similar unanswered question is here (https://internetlifeforum.com/general-webmaster-discussion/1571-webmaster-tools-decreasing-number-indexed-pages/).
My guess that the reason may be that google applied some new policy to remove low quality content from their index or the page text is dumplicate with other page or website or the page got noindex attribute.