why in the world an object has stopped being a referance type

https://imgur.com/a/Fwk7KY5

so… there is an empty object
there is sum to sum up numbers, i use it because its referance type.
than there is the method, it all worked fine.
now it stoped being referance type.
dont get it why, it all worked fine.

as you see it shows 40, than 0, than 15, than 40 again.
while it should be, 40, stay 40, 55, stay 55.