There is a lot of variation in bore diameter in various 9mm pistols, especially ones of foreign manufacture. However, you are probably looking at jacketed (.355) vs cast (.356)bullets. .355 is the "standard" diameter for 9mm bullets. Generally speaking, cast bullets will be sized 1/1000th larger than the jacketed version. If you're seeing .356 jacketed bullets, it's for guns with a larger groove diameter, or for use in .38 Super.