I suspect in many cases the subbing is done translating off the original script, while the dubbing is done using a completely different translation designed to flow better when spoken.
To make matters worse, sometimes shows are re-dubbed [0], but may retain the subtitles that were accurate to the previous dub.
[0]: For instance, Neon Genesis Evangelion was released on the Italian Netflix with a brand-new dub, which caused quite a commotion: https://comicbook.com/anime/news/neon-genesis-evangelion-net...
Side note: in the dubbed version, it's hilarious when they switch to speaking bad English where they would have switched to bad Spanish in the original.