Fonts look different on different iOS versions

I am building an application that will run 开发者_如何学Pythonon iOS versions 4.0 and above. At some point i use UIFont for some labels in the following way:

UIFont *labelFont = [UIFont fontWithName:@"Helveticaneue-bold" size:14];

The problem that i face is that on iOS versions 4.0 and 4.1 are smaller than on versions 4.2 and 4.3

Any idea why this occurs and how i can resolve it?

Thanks in advance