SRTM Plus calculates differently depending on zoom

JSL

I've been trying to calculate some slope stats on lines by connecting to SRTM Plus V3, and I've discovered that I get different values depending on how far in or out I zoom on the line.  For example, if I zoom in, I get an average, minimum, or maximum elevation value similar to that derived for the line from a downloaded static SRTM tiff, but if I zoom to the global extent of the SRTM PLUS data, the values increase significantly.  Is this a GM thing, or inherent in the streaming SRTM? 

You should be able to test this by creating a line, calculating the slope stats, then zooming out to the global extent and recalculating.  Needless to say, this makes SRTM Plus V3 unreliable for anything but close up calculations.

Thanks for any feedback...