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Here's a good reading about elevation gain calculation which lets us understand why a) it is not trivial and b) why different services deliver different values.
https://www.gpsvisualizer.com/tutorials/elevation_gain.html

Best way as a user is to use a consistent data source, consistent usage of flattening/correction... same software when comparing elev. gain.

I guess you do not mean altitude itself, but the total altitude ascend, which is computed from the recorded altitude profile or from SRTM data for planned route

Estimation of the true altitude ascend is tricky. There is no a priori way to determine the just proper level of filtering. It is always balancing of filter oversmoothing of real details with leaving residual altitude noise due GPS or SRTM artefacts.

It is quite common Locus planner comes with up to twice value, compared to Brouter value based on used built-in filtering.

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