OpenACC result changing with size

Question

I am trying to parallelize a c++ code that uses several arrays with the same size. I have a "host version" which is not parallel and a "pragma version" which contains pragmas.

With small size, the values of both versions (host and pragma) are the same. But with a great size, the values are really different. The result of the algorithm is supposed to converge, and it not the case of the pragma version.

Have you an idea of the reason?

Thank you