After upgrading to the findbugs plugin 4.13, all "normal priority (p2)" issues are now categorized as "low priority". I have no P1 issues, but I assume they would show up as P2 in the hudson report.
The findbugs report XML is built with the official findbugs 1.3.9 version and lists the issues as P2 instead of P3.

My guess what happened: You base your plugin on snapshot builds of findbugs. Perhaps the findbugs team decided to change the priority indicator from 1..3 to 0..2, which would lead to the symptoms I observe.
Here is a line from the report XML that shows up as P3 issue in hudson and P2 issue in the findbugs HTML report:
<BugInstance type="RV_RETURN_VALUE_IGNORED_BAD_PRACTICE" priority="2" abbrev="RV" category="BAD_PRACTICE" instanceHash="8121cc4d4cb2107f0be2286da704c324" instanceOccurrenceNum="0" instanceOccurrenceMax="0">