It's not possible to compare ascent calculations from different sources (maps, GPS devices, other applications), as they all have different methods of calculation. There is no "correct" or "most accurate" figure for Total Ascent - it depends on the parameters used. For example, how frequently elevation readings are sampled along the route, whether small changes in elevation are ignored, whether any smoothing is applied, what level of rounding is applied and what form of interpolation is used between sampled readings.
If you want to compare the ascent for different routes you should try to use the same source all the time, as this is the best way to accurately compare the hilliness of your routes.
We have a number of detailed guides giving instructions on how to use many of the features of plotaroute.com.
We have a number of video tutorials demonstrating how to use key features on the site.
Take at look at our Forums for more help and advice.