Stats with Sugarcube.

I've decided to undertake making a pseudo-sandbox RPG, so far I have been able to wrap my head around most of the coding I have attempted but I just can't get addition working for the stat system. Each time I test the stats rather than raising the amount, the number that is suppose to be added is placed at the end. EG. Jog makes stamina to 5025 rather than raising it to 75. The test code I am using is this;

<<set $Stam += 25>>

What I find especially frustrating is

<<set $Stam -= 25>>

does exactly what I want it to. Any help with how to properly get it adding properly is greatly appreciated.