^In the cited service sector where it used to address customers it doesn't imply that they are above or below you. It's simple away of being polite, English as a language evolves, and words gain new meanings whilst retaining their existing meanings.
So Sir means
An address to a superior military officer
An address to a male espically if his name or proper means of address is unknown
A man of higher rank or position.
When used in the service sector by an assitant to address to a customer they are using the second defination. It does not imply in that situation a person of higer socail standing.