Too many extra samples at the end of the transmitted packets


I am using narrowband example to transmit packets. Each
single packet is separated from its neighbors by a second.
The parameters I am using: -m gmsk -s 4000 -r 1000000 --tx-amplitude=1 -f
2.402G -A TX/RX --tx-gain=25 -m gmsk -r 1000000 -f 2.402G -A RX2 --rx-gain=0

At the receiver side (different USRP device), I am converting the
complex stream into magnitude_squared then into a file sink. What I
should receive according to parameters above is a packet of 64000
samples or a bit more (for preamble and synch issues). This is because
sps (sample per symbol) equals to 2. (4000 * 8 * 2 = 64000). However,
when displaying the content of the file sink on a graph, I can obviously
notice extra ~31500 samples!!! What could these extra ~31500 samples be?
Please have a look at the attached image.
I tried different packet sizes, modulations, etc, but there are still
extra ~31500 samples.



I have also noticed similar problem for my CSMA project.