I noticed that the reputation on this page http://stackoverflow.com/users?tab=newusers is not bold, but it is on all the other tabs under Users.
Is there a reason or is it a bug?
I wouldn't call this a BUG. This is more of a design issue for StackOverflow Meta. Simply adjusting your HTML or CSS for it will make it more visible, which I don't even really think is necessary, or be considered a top priority, but I do see how you can view it as an issue.
EDIT: Since it is not consistent, it should be adjusted to reflect it's siblings.