Training deficit calculation and XATA recommendation

Apparently by using the planner and playing with the time of the day, it will display the XSS deficit for the tome of the day that one has entered. As an example, my defict displayed for 12 PM, was 40. However if I keep changing the time of the day, the xert server never recalculates and keeps it at 40 until 19:00 when it changes to 86. So, if I start a workout at 6PM with the 40 xss deficit and exexute a 40 xss workout expcting to go to zero, that will not happen because at 7 PM , finally xert recalculates the deficit to 86. After I download from my Garmin, the XSS defict will be 46, therefore the conundrum. However , if Xert calculated the deficit continuously, at 6pm the " new" calculated defict would hav been about 80 and the recommended workout would habe been a workout of 80. Then, upon downloading the defict shown would be 6 xss and not 46. So, I think, for the time being hoe I can solve this big confusing problem, but the REAL QUESTION IS: Why doesnt Xert calculate the deficit continuously! it makes no sense. Please reffer tomthe graph attached. Thanks