I don't care, and I can only imagine it's very much [status-bydesign] to preserve some real estate (though larger numbers use a smaller font anyhow). Still, just in case it matters, especially for downvotes:
When the number of up or down votes exceeds 99, then the leading plus or minus sign is not shown in the vote split:
The sign is also "missing" in the Ajax response. For the above images, the JSON results:
{"up":"+99","down":"-3"}
{"up":"100","down":"-1"}
and:
{"up":"1147","down":"107"}
If the JavaScript code ever relies on the minus sign (like it once seemed to do) then in theory this might be an issue.