Why does use the pinyin rules for 个?

个is a量词，before nouns，一个人，两个人

个 used as measure word/adjective/noun, the pronunciation is gè

自个儿 (zìgěr), in this phrase, its gě.

It's a tricky question.

Normally you can think when 个 is a count word together with numbers like 一个，两个，三个，四个，etc, it should be in fourth tone. And when there is no number word attached to 个，like the example you provide, xxx是个xxx，it should be read in 轻声。

Though in spoken Chinese, 个 is kinda always tend to in 轻声, even it marked as fourth tone in pinyin.

I'm sure there may be some corner cases which not apply, but I don't have that off-hand.