On Wed, 08 Jul 2015 20:59:24 -0400, wrote:

Generally "non standard" sizes are used where something happened in
the planning that precluded the use of a "standard" door.

Happened in a rental property of ours. I went to replace a damaged
door from a renter, took it out, looked for a pre hung - then found
out it was none standard. A local door factory built me one in their
shop.

I don't recall there being any added expense.