What you are seeing is indeed correct.
Let me know if this example describes your situation, and perhaps the misunderstanding.
I create a 100gb lun on a 200gb volume.
The filer shows: 100gb in use, with 60gb avaliable and 40gb available in Snap Reserve
From the Windows host, I show 100gb available in my lun.
If I were to go increase or decrease my Snap Reserve, the amount of space available in my volume will indeed change (to be more or less)
However, within Windows and within the LUN the amount of available space will remain the same.
The lun is only as large as the space you specified, and will remain that maximum size usable (100gb lun will only ever show 100gb max available space in windows) - Regardless of the size of the volume it sits on.
At which point, if you want to increase the size of the lun, you can use the lun resize command, and manually extend the LUN within Windows.
Hopefully this helps with what you're doing Thomas,
Christopher