[TECH] why is ((unsigned long)a_ptr+(unsigned long)b_int) not equals (a_ptr+(unsigned long)b_int)

I was looking at some code today on SGI super computer, and wanted to make sure that there are no 32-64 bit problems in the code since SGI was super computer. I found the following thing to be interesting

<---------------------->
((unsigned long)a_ptr+(unsigned long)b_int)!=
          (a_ptr+(unsigned long)b_int)
<---------------------->

That is if I cast both the operands of binary + to (unsigned long) the value I get from that addition is different from if I cast just one operand of binary + to (unsigned long), whats the compiler doing here? since if both the operands to binary + are unsigned long and the value(no.of bits) could be larger than sizeof(unsigned long) is it the reason why I get different values here?, I couldn't find this in FAQ.