You need to understand that TDEE (total daily energy expenditure) is merely an estimate of how many calories your body burns with the level of activity you are performing. There's a couple things that don't make sense for the TDEE numbers you posted:
- Your Basal Metabolic Rate (BMR) is only ~1900 calories, but your TDEE is over 2800?
- Light exercise and 15 minutes walking daily is little better then "sedentary" when you are figuring out your activity multiplier.
- My estimate is that your TDEE is closer to 2200 calories if your BMR was estimated correctly
- Considering even that mistake, it's quite possible your estimated BMR is too high as well.
You'll see activity multipliers that look like this:
- Sedentary = BMR X 1.2 (very light or no exercise, desk job)
- Lightly active = BMR X 1.375 (light exercise/sports 1-3 days/wk)
- Moderately active = BMR X 1.55 (moderate exercise/sports 3-5 days/wk)
- Very active = BMR X 1.725 (hard exercise/sports 6-7 days/wk)
- Extremely active = BMR X 1.9 (hard daily exercise/sports & physical job or 2X day training, i.e marathon, contest etc.)
However, almost everyone thinks they are working harder than they really are. For me, weight lifting 3x a week should be using "Lightly active". It's only when I'm doing conditioning and lifting that I should even consider "Moderately active". Most people don't have the heart to approach "extremely active" levels. If you train like Michael Phelps you might consider it. With the level of exercise you described in your question, I would use "Sedentary" for your TDEE calculations.
Estimates only get you close
Your body is a complex system, and when it is operating properly the old "calories in vs. calories out" model works amazingly well. However, if your body isn't working properly, then you need more drastic changes to the way you eat or get those issues fixed.
Common hormonal causes of weight gain include:
- Thyroid gland isn't working properly. The hormones from the thyroid regulate the calories you burn. Make sure you have enough iodine in your diet to keep the gland happy.
- Leptin response is diminished. Leptin is the hormone that signals that it's time to stop eating.
- Insulin resistance. When you don't put enough demand on your body through exercise, and eat too much, your body can't store the energy in your muscles as glycogen any longer.
- High levels of cortisone. Stress causes many bad things to happen in your body, among them increased levels of cortisone. This increases fat storage, and decreases the body's ability to build muscle.
There can be any number of other issues that throws your body out of efficient working, including several diseases, pregnancy, etc.
Most often the cause is bad estimates or bad counting
It's worth using a tool like "My Fitness Pal" or "My Plate" (livestrong) to tally up all the calories, fat, protein, and carbs you eat. Log every last thing that passes your lips. If you eat one mini Reece's peanut butter cup, log it. Make sure your portions are correct when you log it as well.
Most of the time you'll find that you are really eating more than you thought you were. Pay attention to labels. You'll find more often than not, a serving size is smaller than the item you purchase. I've purchased salads only to find out the serving size was not the single salad bowl I purchased, but only 1/4 of that bowl.
You aren't that far off. Gaining a pound a month would be perfect for strength training. Dropping a hundred calories off every day will be enough to even it out so you aren't gaining or losing.