Description: TrailRunner downloads elevation data from a source that originally retrieved the data from NASA satellite measurements. What NASA did was to scan the surface of the earth and to record the topmost spot found during the recording. This could be the street level surface, a mountain peak or -- and that's probably what you are facing -- the top of a house or tree.
Solution: The error described comes from false GPS recordings. If your recorded GPS point is slightly off track due to a measurement error of your GPS, then the wrong elevation data point is being downloaded. Unfortunately there is no solution to this problem. Just ignore the spots where this had happened or correct the values by hand.
Hint: To correct values by hand, open the black info panel and manually enter elevation data value for spots that seem to be inaccurate.
Description: Elevation is the absolute difference between the highest summit and the lowest valley. It may also be named altitude difference or altitude range.
Ascent is being displayed as ascent/descent value which is the gain in only positive / negative altitude differences.
Example: With an altitude series of 10, 13, 15, 13, 11, 16
the elevation is 10..16 = 6
the ascent is 10..15 + 11..16 = 10
and the descent is 15..11 = 4
Tip: As most GPS receivers tend to be very inaccurate with the recoding of altitude data, the ascent and decent values can become unnaturally high as only small gains can sum up to huge values. In TrailRunner > Preferences > Advanced you can set a high pass filter for the ascent/descent calculation. See Filter altitude noise.
Additional note: It may even happen that you record a round-trip and the ascent and descent values differ. This comes from the simple fact that altitude recordings made by a GPS are extremely inaccurate. To calculate the altitude, the GPS requires good reception of at least three satellites. If reception differs during two visits of a location, the derived ascent and descent values will also differ.