There are generally higer monetary levels of +++ when a 'o' is involved because additional components are payable - rarely do any 'drop off'.
So if booked upfront as 'o's then with date/time changes the following often happens:
"x" to o - since booked as o originally, +++ is already correct. Often the agent will simply make the change as a re-validation.
o to "x" - easier for for agent to simply re-validate and not bother calculating refund of +++.
(Of course, if there's a UK 'o' in there, the APD can be rather significant and a refund may be desirable)