ls sorts filenames strictly lexicographically, comparing character by character, so e.g. "055436307" is compared as the characters "0", "5", "5", etc. so it sorts before "121134" because "0" is less than "1". if all compared characters match and one string ends, the shorter one comes first. Symbols like _ are just more characters, and their position relative to digits depends on the locale’s collation table. Google D…
That said the author's situation where it's numerical and different lengths seems likely rare enough that it probably isn't worth complicating things.