I have a mate who carries out basement digs on this basis at times,and I believe the percentage is tiered as an incentive to keep costs down (I.e. 15% to £150,000 dropping to 10% thereafter,just as an example.)
At first glance it might not seem like much of a carrot but bear in mind that if he ‘milks’ the job & breaks the threshold then,come the end of the job,he’s still tying up all the annoying loose ends,but earning a far smaller commission for his trouble.