I don't think the urgency field is meaningful in Ubuntu, especially for post-release fixes - I believe a package migrates as soon as it's approved and passes builds/tests. The urgency field comes from Debian, which uses it to describe how many days a package should sit in Debian unstable before migrating to Debian testing, in the hopes that if it's buggy, people will file a bug before it migrates to testing. The Debian default is now "medium" (5 days) instead of "low" (10 days), but people with older tools tend to generate changelog entries that say "low". (And even in Debian, I don't think the field has any meaning for post-release updates; I think it only applies to unstable-to-testing migrations.)