Why haven't I lost more weight?

So a few months back I weighed in at 100kg, and I was expecting to lose at least 5kg, perhaps as much as 10kg. But I've got down to around 96-97kg, and haven't budged from there. A bit baffled.

• I've burnt loads more calories through exercise in the last few months than I've done for a very long time.
• Unless I'm in complete denial, I don't feel that I've increased my calorie intake over the last few months. Indeed I reckon if anything I've reduced it. 
• I've (almost completely) cut out chocolate for the last month'ish.
• I was a moderate drinker of alcohol in the first place, and I'm purposely drinking even less now.
• All my coffees are "skinny".
• I normally eat a healthy, high fibre breakfast with fruit, typically more fruit for lunch, and a good home cooked meal in the evening (thanks wifey!).

Bearing in mind I'm no expert in this area, the only rational explanations I can think of are:

• The old "muscle weighs more than fat" reason, i.e. the effect of my running has been to increase muscle relatively more than it's burnt fat.
• At my age (45) muscle mass/density is a lot less than when I was in my 20s, and you need muscle to burn fat, therefore with less muscle to do the job, burning fat requires a lot more effort than it used to.