The sensor is retrieving its data from the SOAP-API. The data regarding Datastore latencies can't be compared to ESXTOP directly.
You can see the data in vSphereClient -> Performance-Tab -> Advanced -> Switch to: Datastore.
At the moment PRTG shows a rolled up average for all available datastores for the host or vm. We are currently rewriting the VMWare-Sensors to get more performance as well as new features. One new sensor-type will be a dedicated datastore-sensor that supports more detailed informations about latencies.