Jean:

That is happening because of a setting in the Company Profile which forces all Customer codes to be the same length (6 digits/characters in your case). Shorter codes are padded to the full length (with periods in your case). Having all Customer codes the same length is a tremendous benefit when it comes to searching by code.

So before you make any changes, why do you want those Customers to be at the top? How does that help you? How are they different from other Customers?