1″ does not equal 1″

tyrsu

Hello!
I have a program that is meant to run for 40 minutes, however I was finding that I would set it going and come back 40 mins later and it would still run for a further 8 minutes.
So yesterday I sat next to it with a timer and found out that for every 1″ that the program counted, 1.2″ would pass in real time, resulting in an extra 12″ every minute, and an extra 8 minutes over the whole program.
My session time code is was the following:

S.S.27, \Session Timer
S1, \start session timer @ START command
#START: SET T = 0 ---> S2
S2, \increment every second
1" : ADD T ---> SX

I have currently fixed it by doing the following:

S.S.27, \Session Timer
S1, \start session timer @ START command
#START: SET T = 0 ---> S2
S2, \increment every second
1" : SET T = T + 1.2 ---> SX

It is working now but I was wondering if anyone has come across this before or knows what is causing this?
I also tried to fix it using the following:

S.S.27, \Session Timer
S1, \start session timer @ START command
#START: SET T = 0 ---> S2
S2, \increment every second
0.8" : ADD T ---> SX

But that code counted accurately so my time count was off by -0.2 seconds per second. So something is going wrong at 1″ specifically.
I have attached the code as a .txt file if more context is required, (for some reason this forum doesn’t allow .mpc files – I tried to upload one and it deleted my whole post).
Many thanks for your time and help,
Suszie