Amateur Pace Question...

Hello, I love swimming as part of my workout routine and am relying on it more and more because I feel great right after. I am swimming a 2000-2500yd workout 4-5 times a week and doing cardio machines other days. My 50yd pace almost ALWAYS comes out to 1min. Fastest/slowest were :58 and 1:02. My first 50 tends to be around :40 or slightly less. I am really going almost full out the whole time but I have had no training and never competed. I noticed when I look at "Calorie Burned While Swimming" charts that this pace is considered light to moderate. This has pretty much always been my pace and been consistent since August. My question... I feel like I am exerting Fast/Vigorous effort, not Moderate. Is my pace really THAT slow? Isn't it more about the effort than the pace? I am 6'0" 250lbs (should be about 185lbs) Thanks in advance for your input.