samtools flagstat reports an odd number of reads "with itself and mate mapped"

[syfo]

How can "samtools flagstat" report an odd number of reads "with itself and mate mapped"? I don't get that...

samtools flagstat reads.bam

161533046 + 0 in total (QC-passed reads + QC-failed reads)
0 + 0 duplicates
137350248 + 0 mapped (85.03%:-nan%)
161533046 + 0 paired in sequencing
80766523 + 0 read1
80766523 + 0 read2
106075436 + 0 properly paired (65.67%:-nan%)
124869771 + 0 with itself and mate mapped
12480477 + 0 singletons (7.73%:-nan%)
8181512 + 0 with mate mapped to a different chr
4410235 + 0 with mate mapped to a different chr (mapQ>=5)

Any idea?
Reads come from a mate pair protocol, sequenced on HiSeq, aligned by bwa.
Thanks!