Once you find the perfect font, with it's microscopic differences set just so, you'll realize you're on the web and not print so fonts look different to your us

Once you find the perfect font, with it's microscopic differences set just so, you'll realize you're on the web and not print so fonts look different to your users. High DPI, low DPI, Windows/Mac/Linux/iOS/Android, etc etc. I wouldn't waste much time fretting the differences between Helvetica and Arial unless you're printing the result.