It’s all to do with sustaining weight loss. Unfortunately, too low below your TDEE can have many problems. Don’t get me wrong, you’ll see fast results quickly and you’ll think it’s working, but internally it’s not great for you, and it’s not great for maintaining weight loss either. There is a limit on how much fat you can burn per day before it starts breaking down muscle instead, and peak muscle loss occurs for most people when they are consuming 20-25% below their TDEE, which at an expected 1800 and doing 1328 today, you’re just teetering over the edge of that. There is also of course the risk of ‘starvation mode’ if you go too low. It’s not as dramatic as people think, and it takes a while and numerous factors, but your body can slow your metabolic rate if it believes it will continue to be starved, therefore it slows down the weight loss, people drop calories even more as they aren’t seeing improvements and the cycle continues, or, individuals feel the hunger, over indulge and eat back all the calories they’d previously reduced. I understand you think it’s high, but your PT will have looked at what you’ve asked for, what you’re doing daily and adjusted your allowable consumption to include all of those factors. There is a science behind it, and they’d have followed it to give you the best sustainable results. There’s no point losing for example a stone in a week through immense calorie deficit, if you can’t keep it off.
Only thing is, if your BMR is 1391 and your doing 2 workouts a day, I’d question if your TDEE is 1800, as that seems relatively low, especially as a Les Mills class can burn up between 400 and 700kcal’s alone (depending on RPM etc, but an average class is 500kcal). As effectively, today you could have a BMR of 1391 + 500kcal for Les Mills and let’s say 200kcal for the gym, that’s TDEE of 2091, never mind your daily walking etc so even if I just add 200kcal for the rest of the day, that a final TDEE of 2291, so consuming 1328kcal’s that day is only 58% of your TDEE (even with no rest of the day kcal added, it’s still only 64%), which is way below what they advise of 75%, whereas your PT’s calculation of 1700 increases that to 74%, therefore with those calculations, your PT’s target of 1700 makes much more sense.
I’ve done a degree module in this (sports nutrition) so I’m reasonably well qualified, also, your PT knows you best and has your best interests at heart (otherwise you’d not pay them if you weren’t getting results), personally, I’d listen to them. The internet is rife with awful information regarding nutrition and dieting, much of which when you study research papers has been disproved or proven to have health issues associated with it. But, most importantly, I hope you achieve your goal weight!