Radicals vs glyph origins (specifically for character 肚 in 肚子)
Look at the middle parts, they are different.
月 = 月 (moon)
肉 = ⺼ (meat)

⺼ - 肉部首
(俗稱「提肉旁」）：CJK RADICAL MEAT (U+2EBC)
簡體將從此形的字歸入月部。

Simplified Chinese characters merged ⺼radical into 月 radical


The '土' in '肚' is the phonics component