Hi breizh22,
this means that Sniffer sees the continuation of a frame, where the first frame of this transmission is not in the captured data. This could be, that you really havn't captured from the beginning or you have different routes to the receiver and the first packet was traveling this alternate path which you didn't capture. i.e. you have two different Routers und you where filtering on the MAC Adress of only one of them.
If you have captured all frames it would look like this
HTTP: CONTINUATION OF FRAME yyy : xxxx BYTES OF DATA
i hope this helps
regards
Helmuth