when you do it that way you are actually mashing some of the liquid at 160f and it changes the outcome as far as mash temps from what the software estimates at a certain mash temp..
the real question is if you want to mash at 150 why is your mash at 140? some sort of step mash or poor strike water calculations would be the only reason this would occur right?
The only thing that matters is the temperature of the wort. The temperature of the mash bed does not matter. If you're flowing hotter wort through a cooler mash bed, eventually that 'wave' of hotter wort will make its way through the entire bed and the whole system is at temperature. It's unreasonable to think that you can instantly step the temperature of your entire system up. It does take time and watts to add and distribute heat.