Hp and AC difference when switching clients??

[arailgun]

I am currently playing on a server that supports Titanium, UF and SoD. I normally use Titanium , but when a guildmate recently pointed out that I recieve more hitpoints using Underfoot I was taken aback and tried this. After switching to underfoot client I noticed I had gained 500hp, i even tested this using #mystats and it showed the server as giving me the additional 500 hitpoints. I was just wondering Why this is and is there any way to fix it, or is it just how the client is coded?

Thanks!