Elevation appears to be a "work in progress" on MapMyRide. I like many of the features, but elevation is the weak point (you get what you pay for, so I can't complain about a free service). This may explain your results - it's from their FAQ on their website:

"Elevation Ascent / Descent seem to be wrong!
We report ascent and descent stats for routes when they become 'significant'. This is currently set at about 60 meters of total climb. We can't guarantee a great deal of accuracy for routes with ascents < 60m, so we don't want to confuse our users by displaying inaccurate data. For long ascents, you should see your stats appear the the right of the elevation profile.

Our ascent and descent calculation algorithms don't count every small elevation change at every point because we need to filter out 'noise' in the underlying data. Prior to our latest algorithm change, we were reporting ascents that were in many cases too high. Our new algorithm gives more reasonable results for a wider variety of routes. However, since we know that this isn't 'perfect' quite yet, we've provided the raw data in CSV format so that users such as yourself can interpret the data as you choose.

Another reason we've released the raw data is so that we can collaborate to improve this feature. We'll soon be adding forums to the site, and we'll have a community collaboration on improving the elevation calculation functions."