I suspected they were buffed. At least after I tested my other theory that the lines between them damaged you if you passed through them.
Maybe instead of increasing their HP, you decrease the damage they take? Since it shows the damage they take, you could see that easily. You could also buff their speed a bit, which would be obvious at a glance.
You could also have them appear earlier. If you had one at the very beginning when you're taking enemies down one at a time, it would be clear that it takes longer for one to fall.