Numbers are always going to be off because each person is different and depending on your activity you could use a lot more calories and require a lot more too. Sure you can average but that's never really that accurate.
I suggest keeping track of what you eat exactly for a month or two 6 would be better and then figuring out what you need yourself.
We keep very close track of our food usage....we have been doing this for years as part our food prep inventory which now sits at 8 months worth.
On a couple of other forums, the calculations from the linked web page were also called into question as to how inaccurate they appear to be.
Like anything else, take what you read on the internet with a grain of salt....or a bucket of salt depending on how much you have stored.