strange behaviour with ctrl+A (increment)

Something odd going on here, and I don't understand: I thought I
remembered how to increment numbers, but it seems not :-{

Here's two lines from a file I'm working on
bar // 04
bar // 05

If I place the cursor on any digit and press just ctrl+A, I get the
expected results
bar // 04
bar // 05

But I actually want to add 59 to each number. So, I put the cursor
first on the 4 and then on the 5; each time, I enter 59 ctrl+A

Result:
bar // 0100
bar // 076

(and by the way - what should I be looking for with :help? :help
increment gives me "E149: Sorry, no help for increment)

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