Just a guess but if a door on a 500 gallon lp tank warps it would be much easier to convert to a squared off door than to try and get the warped door fixed. Another guess would be the door would be huge on a 500 gallon lp tank. Squared off doors could be easier to seal and make stay tight.
Is it normal for one testicle to be larger than the other two?
Many times people have a hard time getting rounded doors to fit properly. This is due to they tend to spring when you cut them. Going with a square door eliminates this. All of our builds from this point forward will have square doors.