Why do some pools require a worker name with an underscore and others require a period? The majority of pools I see that they use a period such that:

username.workername

others use:

username_workename

I see that it definitely is not interchangeable.  How come some pools use underscore (such as bitminter and btcguild)  and others use a period (like gigahash.io)?
These are just arbitrary rules. Made by the pool admin. Really doesn't matter.