The statistic (as used by the actuaries who program Staysure's computers) probably show that there are more claims from people who are travelling as couples than as singles.
For example, many solo travellers will be on tightly-scheduled business trips and therefore less likely to visit nightclub areas (where travellers might be robbed) than couples on holiday would.
Similarly, because most solo travellers are probably on business trips, they're less likely to drown while swimming in the sea or to fall off a hired motorbike.
Trying to make sense of the way that insurance premiums are calculated though is almost impossible for the layman. My best friend at school went on to obtain a first class honours degree in maths before commencing his studies to become an actuary. He gave up after 4 years because it was simply too hard for him.