App data usage

Nexus 6 with Lollipop 5.0.1
version of Xiialive: 3.3.0.3

I just bought the pro version of Xiialive. Sofar I'm liking it quite much, but there are some niggles.
Most important one is data usage. The 'About' screen shows some statistics like data usage and total playing time. According to the app I've been listinging to music for 75 minutes. The data usage for this 75 minutes is 70MB; I've only been listening to 128kbps stations so this seems a bit high (128kbps should be around 43 MB per hour unless I'm calculating wrong) but still acceptable. Unfortunately my provider tells me that I've been using 248MB of data. My phone then tells me that 99% of this traffic is caused by Xiialive. This is quite some way removed from the 70MB that the app specified. Do you have any idea what might be going on here? I'm using the app in the car a lot, so using WiFi is not really an option