Daniel Witherstone
It's tricky without seeing but the only reason I can think of is that sometimes with older toilet installations, the soil drain going through the floor is further into the room to accommodate an old s trap type toilet pan. Sometimes this drain pipe can be difficult to alter under a floor, especially if it made of clay not plastic. Even still, if this is the case, you can construct a false wall or boxing behind the toilet to alter the measurement for the the waste outlet rather than altering the pipe. Other than that, without more information from you it is difficult to give a reason. I only presume that this is the case with your current toilet set up.
