Q:
Toughness Doesn't Add Up
A:
This is the wrong forum to be asking this question; the general forum would have been a better place. However I’ll answer it b/c it’s easy. More goes into toughness than just equipment; you have a base toughness before even adding equipment values(0) and then there are several other factors like skills, traits, and consumables(1) that also have an effect. Depending on you build, your toughness (and most other stats) might actually fluctuate during combat for these reasons.
(0) https://wiki.guildwars2.com/wiki/Attribute#Primary_attributes
(1) https://wiki.guildwars2.com/wiki/Toughness