[Radek]

Bungleu has explained the phenomenon. Now, it should be clear what's happening.
174 HP is too much for an assassin at level 22. Every time you level up, your HP are rerolled from scratch (for examle at level 23, the game rolls assassin's HP 23 times and adds the results together) and because you have too much HP, the rolled number of HP is almost always less than your current number of HP. Therefore, you get 1 HP during leveling up.
Suppose now that you are getting 1 HP all the time up to level 38. That's 16 levels, you will have 190 HP and level 38. It's time to score some successes during leveling up: the average HP per level for an assassin is higher than 5 so that the "roll from scratch" method should give more HP than your current number of HP sometimes. Therefore, you get more than 1 HP during leveling up.

Notes:
1. Every HP boost in the game is worthless. Well, you get some HP now but it means that you will get 1 HP per level later - until the boost vanishes. Therefore, ignore your HP and concentrate on your ability points. When all your stats become 18 or more, stop "controlling" your levels up (except for training opporunities, of course). It's no use.
2. The "roll from scratch" system implemented in the game behaves the following way:
a) You will get 1 HP per level until your HP are much lower than they should be.
b) Then you will get bags of HP suddenly - for example 70 HP or even more. Your number of HP will correspond to your level much better now.
c) Go to a)
In the other words, your number of HP is usually lower than it should be. When the number of HP becomes unbearable, you get a "bolus". Then your number of HP starts lagging behind your level again. Then you get another "bolus". And so on. Oh, well...