That calculator is inaccurate (in my opinion).
I tested heavy crossbows this morning with a stopwatch, and there is no difference between 50ssi/120 stam or 45ssi/150+ stam.
The knucklehead, and the stratics calculator, and the charts on this site all say I should be swinging a heavy at 2.5 seconds at 120 stam with 50 ssi, but I only swing at 2.75.
Composite bows tested correctly to me.
I've always used the Athene charts on this site, and it may only be the 5s weapon (heavy crossbow) that is off, but I was putting together a new suit for an alt acct (since I got slapped down by EA) and got in the mood to test it out since I was trying to decide on whether or not I wanted an ssi ring + a ranger cloak for this suit.
I have no idea if any other swing rates are off, and won't be testing them.