On my desktop and mobile apps, it says my number is 555-555-5555 ext. 1000. Online it says my number is 555-555-5551. Why aren't these the same?

Every user has access to two numbers: the business number and a personal number. Your business number, the one shown in Manager Express in the top left, is your main company number. Your personal number, which you see in 8x8 Work for Desktop and Mobile, and in Manager Express on the Users page, is the number callers use to reach you as an individual user.