This is probably related to your project density. For each project, the density only needs to be selected once by the designers, so that they can pick the “resolution of the design” that they are working on.
Let's say you're working on a 640×1136px iOS artboard. In this case, you need to select 2x as your project density. Then, if a measurement is 20px in Sketch, Zeplin will display it as 10pt to help developers out since they use 1x values while coding.
If you think that this is the issue, you can change your project density from the project's "Dashboard" tab, right panel. After changing it though, make sure to re-export your designs since Zeplin generates all the assets based on this setting as well.