Could be that they are working as well and are also taking their tax credits into account. I receive housing benefit, also work and get tax credits, as well as maintenance for the children, so am able to afford the rental here.
Oh right I was thinking finding £150 extra a month sounded like quite a gap TBH and that it could be a bit fragile to rely on them finding the extra themselves. I don't know the benefits system very well though, any limits on earnings that apply etc.
Perhaps with a combination of HB and earnings then they are potentially more secure than someone relying on earnings alone then.
If you were working and kept your costs low £150 would be ok to find.
If your tenant was (for example) a mother who had recently separated from her DP/DH and was being paid maintenance, the maintenance that she received would not be taken into account when calculating how much housing benefit she would receive so she could use it to cover the shortfall.
The only sticking point would be if someone was earning and receiving Housing Benefit as a top up and then lost their job, topping up £150 per month while you were living on a JSA budget could be tricky.
The other tricky thing is that the HB allowances have been lowered so much that in a lot of areas it is near impossible to get a full size house (that the family would be entitled too) on the benefit allowance. Have you looked on right move to see how many 4 beds in the area there are for £599?