That is exactly what i didn't understand. In my original post, i noticed the exact same "issue". As i said, i have been away from the viper for quite some time, more than a year i think.
Before that, i can not exactly specify when, i could create a new SPI, and if i remember correctly, its delta didn't carry over to the "non current steerpoints".
In example, i had some targets in the area of steerpoint 4, engaged them with mavericks via tpod and could fly the rest of the flightplan, like stp 5-10 without any shift in position or at least i didn't notice.
What really makes me wonder is, it seems to only affect the HUD and EHSI, the HSI MFD page seems to be unaffected somehow.
Im not saying that this is not how the real thing works, but to me it seems somewhat counter intuitive.