How can changing the bitness of an application assembly affect its operation?

Question

Programming

Hello. There is a C++ application using Qt that reads something and draws a graph using QCustomPlot. I program in Ubuntu, I made the first assembly 64 bit (as well as the bitness of my OS), the second - 32 bit. I got the following: I did not change a single line either in the *.pro file or in the sources, but the graph in the 64-bit version differs from the graph in the 32-bit one. The input data in both cases were the same.
Prompt, please, in what there can be a reason. Thank you.

Answer 1

20 traps of porting C++ code to a 64-bit platform