What to do when you are suspicious about numerical results in a submitted manuscript that you are reviewing? I am currently reviewing a paper and feel suspicious about their numerical results. I feel their results have been slightly modified to exhibit their superiority compare to other comparative methods.  

I cannot give an exact mathematical reason why I think their numerical results have been manipulated, but it is so odd that in all 15 scenarios they have been better. Actually, I am pretty sure that their method is better than his benchmarks but not always. Maybe they modified some scenarios.   

As I don't have a mathematical reason why I think some of their numbers are altered, should I ask for their computer code and run them myself?
You say that you don't have a mathematical reason to be suspicious. But have some reason. You should formulate this as a reason in your report as well as you can.

Moreover, I think that it is totally reasonable to ask for code and data to reproduce the results. Probably the journal has some policy about reproducible research and you could refer to that.