I wrote an exam about this stuff yesterday.

In J's equality is usually checked in a way that variables are casted to the type of the other one. "25" == 25 evaluates to truey because the string converted to int is equal to the int and the other way around.

You can however check if the thing is identical, using "25" == 25 which skips type conversion and would evaluate as false.

I assume the same thing happens here, null is casted to int, which gets the value 0.