When the zerker's cloth broke you lost those mods. You must have had more HP+MP than your base which is why it left them full. If you had 10/2000 HP/MP then it would have kept you at 10/174 (whatever your base HP/MP is).
Does that make sense?
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This is a good question though. When an item breaks it should probably reduce your MP/HP by percent. So if you have 1500/2000 HP before the zerkers cloth breaks you'll end up with 75/100 hp after it breaks.
while that does make perfect sense you're forgetting that the ONLY mod zerkers gives is str. Not Hp.
EDIT: Also, it healed me for about 1500, pretty sure there's not an item in the game that gives that much.