HAL is decidedly second-tier. Given the option, everyone would pick arXiv over HAL. Hence, HAL hosts lots of stuff that didn't (even) make it to arXiv => lots of subpar dredge.
Turns out that "better" for many people means "better moderated", since static hosting is hard to differentiate. And at present Arxiv is winning that one (at the expense of considerably higher running costs due to said moderation)
It is actually quite common to come across HAL in subfields of mathematics in my experience.