I don't think 1.5" is a major issue in itself, certainly not health-wise. It's only if it's 1.5" every month...
If you've been doing a lot of ab work that can add a little bit, not more than an inch but still. If you see this anatomical drawing,
the obliques there are typically very weak on an untrained person, and can actually slope in; when built up, they take up more space. So if you've kept the same body fat total but done a lot of ab work, your waist measurement can actually increase a little bit. But as I said, that's probably not the full 1.5", but might be 0.5"-1" of it. Or if you've done little ab work, 0"
Caliper tests you want to take with a grain of salt.
One of the assumptions behind the caliper test is that fat is spread evenly over your body. And we all know that describes almost no-one. You touched on this mentioning that your own was concentrated in a couple places. That's the reason most caliper tests choose 10 points - on men, chest above nipple, midaxillary (under pec towards back), bicep, tricep, suprailiac (hip), subscapular (under shoulder blade), lumbar (lower back), thigh and calf; on women, the tricep is omitted - not just 4.
With 10 points, I've seen the same guy caliper tested by three different trainers, and it was +/-5%. With 4 points, it was +/-15%. The scientific sources tell us that the best that can be achieved is +/-3.5%, simply because of wonky calipers, different brands having different pressures, amount of water in the person varying, and so on. So a 100kg man could lose 3.5kg of fat and gain 3.5kg of muscle, or vice versa, and the caliper test mightn't tell you that - but with the extra weight you could lift, and a mirror, you could tell.
At best, if you took the test every week or so and graphed it, the inaccuracies would shake out and show you an overall trend - but only if the change were greater than the inaccuracies, like from 35% fat to 10% fat, whereas if it's 20% to 15% you couldn't be sure.
The real test is how healthy and energetic you feel, and if you're happy with your appearance. I know that's hard to put numbers to, though, and numbers make things easier.
How easy it is to cut off the extra kilos varies a lot, in my experience. But if you're bulking up with muscle so well, you have got the muscle tissue to eat up the fat, and you obviously have the discipline to do the cardio and/or change your diet, so I'd be surprised if you had a lot of trouble. The ones who have trouble are usually those of low muscle mass and without discipline.