How can I explain this difference?

Is that plausible?

Differences in the pushes in the matelso Control Panel and the conversions in Google Universal Analytics may result from Google’s attribution model. For example, we send 2 pushes if the same caller calls twice, but Google combines both events into one conversion.

This is clearly visible in our user-defined reports, for example.

Under Overview, you can see “Unique Events” and the “Total Events,” for example.

See also the following article:
How do I find my inbound calls in Google Universal Analytics?