HURCO VM10 - NC PROGRAM CHANGING RUN TIME MYSTERY

Hi, I am new to the forum, but not to machining. One of the machines I am responsible for is a VM10 with WINMAX MILL V8.01.09.25 ST installed.
I have a very strange thing happen with some NC 3D milling programs which are saved onto the machine and run frequently, although often months in between. The run times can vary dramatically from when the same program was run previously. For instance, I am currently running a contoured part that is taking 1 3/4 hours. However, the last time I ran this program it took 2 1/2 hours! A difference of 45 minutes. Yet the program is the identical one to last time(I made a note of the date and time of posting contained in the prog) I have not altered any parameters on the machine so I don't understand why this is happening. It has happened a lot over a long period of time.
When I set up a new job on the hurco, I use a host program that I import the tool library and part set up from. The host program is named 'latest offsets' and I always use this to set up the part position and the tools. Then save it and import the data from it to the next program to be run. This is because the hurco does not have a separate tool library like most machines, and doing this eliminates confused out of date tool data use.

I only have this problem with NC programs, which I generate through OneCNC and post out as Fanuc OM, which are then saved to the control.
Does anyone have an idea what is going on?