QGIS distance between points - line_locate_point()

I need to calculate distances between points that are overlapping a layer with a line (representative of a river corridor (thus there are two layers; one is the points the other is a line). I’ve tried to follow this example (Points layer distance from the start of line layer in QGIS). I get an output, and I think I’m doing everything correctly, but I’m not sure what unit the distance measurement is coming out in. If I use the measuring tool in QGIS I get very different values.

See below:

Here are some screen shots of the attribute table, points and line layer.

However, for example, if I measure the distance between point 5 and 6 using the measuring tool I get a very different answer.

Where am I going wrong please?