Weird variable that only works when the variable is displayed(???)

So I am having a problem right now where code is misbehaving on me.

I have this code that tests whether or not you need to repair the ship in my game:

(if: $HP is $HPmax)[The Pequod does not need repairs.
[[Sail forward|SailForward]]
[[Sail back|SailBack]]
[[Build|Build]]]

It doesn't work. So I decided to add a thing to print out the value of $HPmax because I had a hunch that was the problem.

$HPmax
(if: $HP is $HPmax)[The Pequod does not need repairs.
[[Sail forward|SailForward]]
[[Sail back|SailBack]]
[[Build|Build]]]

All of a sudden it works fine.

I have another case where the same thing is happening (strangely enough with the same $HPmax variable.) This code sets the HP to the maximum amount of HP.

{(set: $HP to $HPmax)
(set: $RepairKit to $RepairKit-1)}
HP: $HP
Morale: $MORALE

This makes $HP show up as 0 every time. But when I add a test to see what the value of $HPmax is, all of a sudden it works.

{(set: $HP to $HPmax)
(set: $RepairKit to $RepairKit-1)}
Maximum HP: $HPmax
HP: $HP
Morale: $MORALE

Anyone have any idea why this would happen? Why would displaying the variable change the value of that variable?

I hope I am explaining this well...