Except that the OP said he was doing an "Admiral Mission"...not just going against wandering encounters.
Anyway, doesn't matter. I've found the threat assessor to be off sometimes. I've taken down things +6 and +7 easier than some things that were 0 or -2. Not sure whay it happens, but it does.
There's another little indicator that pops up next to where the +/-# appears, it's an arrow that either points up or down (or not at all), and when you hover over it the tool tip says something like "this enemy is tougher/weaker than normal". Which may be why in some cases +7 enemies seem easier than -2 ones.
In any case, the difficulty could stand to have a slight difficulty increase, or at least the Romulan BoPs could as I found myself soloing a group of about 7 +1s without feeling too threatened. And this is coming from a casual carebear.