Mystery WSPR signal on 28 MHz....

Over the last week or so, I have at times seen an odd looking signal while using WSPR on 28 MHz. It starts off about 10 Hz too low in frequency and then drifts upwards for about 30 seconds and then stays reasonably steady for the remaining 90 seconds of the WSPR two minute time period.

When I first saw this signal, I wasn't getting any decodes but eventually, it came through.....it was GW7KNF in West Wales on backscatter. I have no idea as to why his signal drifts at the start but it is unique to his signal.

Normally, I think a signal needs to drift less than 4 Hz to be decoded. Considering that at least 20% of his signal exceeds this then perhaps you don't need to hear the full two minutes of the signal to decode it? Going on the above example, it looks about 90 seconds.