Total ascent/descent can’t really be compared across different platforms – there’s some more about this in the FAQ: https://cycle.travel/advice/map/faq. If you were to ask (say) RideWithGPS or Strava or Komoot you would get three different figures again.

Generally c.t takes an approach of trying not to overestimate little differences – you will see that on the valley section from Mauzac downstream, c.t records a steady descent whereas (for example) RideWithGPS says this section alone has a climb of 59m and a descent of 66m. I think c.t’s approach is more accurate here for what is essentially a towpath ride, but of course there will be little climbs here and there (e.g. around bridges) which will be recorded on a Garmin altimeter but not on the elevation grid used by routing sites.

Tim, you may find it useful to read the article about calculating route total elevation on Adam Schneider's GPS Visualizer website. It explains a bit about why this is difficult.

https://www.gpsvisualizer.com/tutorials/elevation_gain.php

I think that cycling computers (a Wahoo Roam in my case) tend to over-estimate the elevation gain, because they add in every bump in the road that is arguably not "real" elevation gain. Chris