I had a similar problem back in the day when I applied for a job with a small, local payroll company. The MD was a nice guy and a small staff of a dozen long-term people. He figured the best way of seeing if I'd be "a good fit" would be to set me a task and go from there. Fine .. I did that and he set me another which was more payroll-business specific. Again, I did this, but then he came up with another one, and I realised he was getting me to develop his next project for free. I declined and made a hasty exit.