Grade is really hard for a GPS to measure. First, the ability of the GPS to measure - at best - is something like +/-13 feet. So that can translate into a big grade difference. What Garmin does is have a barometric sensor in the GPS to help figure this out but it's also not perfect and is subject to changes in air pressure. It helps but still not great.

So, since grade is expressed as rise over run and as a percentage, that 13' variation can be a problem. If you are in difficult GPS reception territory and that +/-13' turns into something much large (common), then the number is meaningless.

I suspect that's what's behind the problem. The GPS cannot get an accurate enough vertical position fix and they'd rather not report a number than something wildly inaccurate.

J.