"evalf" output depends upon style of input?

Very happy with the output of the following line:

But I'm confused about the output of the next line. Is it a limit to the calculation or a display problem?

and yet this next output looks fine:

Fortunately, there appears to be no difference between x and b:

But these next  lines suggest there is an actual limit in the calculation of a.

Note - when Digits is set to 30, the calculation difference between x and a disappears.