The problem is that they didn't allow for the extra centimetre of overlap. So it didn't fit correctly.
make a possibility or provide opportunity for; permit to be attainable or cause to remain; "This leaves no room for improvement"; "The evidence allows only one conclusion"; "allow for mistakes"; "leave lots of time for the trip"; "This procedure provides for lots of leeway"
If you allow for certain problems or expenses, you include some extra time or money in your planning so that you can deal with them if they occur. You have to allow for a certain amount of error