Yes there is an obvious reason/s for this.
What happens with all machine that have a countdown timer is that the time is altered depending on the fill pressure, load size, distribution and water temperature.
What you see at the start is the optimum if all those conditions are correct.
Most machines (not all but I know ISE10 does it) is to recalculate on-the-fly as it determines the various conditions at points within the program. So, for example, if more water is needed for a larger load then it fill more, which takes longer, has to heat more water, which takes longer... and so on. And, obviously, things like having to redistribute for spin, cannot be calculated until the rinses are complete.
By watching the countdown live and noting the changes what you are in effect seeing is the machine recalculate on a load-by-load basis and they will often not be the same. In fact, I suspect that no two loads would be exactly the same.
It's not inconsistent as such, it's based on a number of variables calculated in real time with the machine compensating for anything that it determines is or could be a problem for it. There's several reasons for this but it's primarily to protect the machine and the clothing.
To put it into a simplistic analogy think of it this way, I can use Autoroute to calculate a route from London to Manchester and it tells me that it will take 3 hours or whatever. What it can't foresee is road works, accidents, go-slows and traffic along the way, therefore the initial "estimated" journey time may well not equal the actual traveling time. But, under optimal conditions, the initial estimate is probably pretty much spot on.