Yes drop in care spots are generally more then part time and the reason is because part time is steady (predictable) income weekly whereas drop in isn't . If the provider is keeping that space open for drop in there maybe some weeks she makes nothing on the spot. If you have found that there are very few providers who offer the service that you need and the provider you have is good and your child is happy and thriving then sometimes it's not all about the money which by the way she is actually trying to save you some money since part time is cheaper then casual.
When I have a rare spot come available and I have inquiries and the parent says they just need part time I tell them the spot is $200/week regardless of how many days they use. I only offer part time for special circumstances. For Instance I have a family currently under contract that is due to have a baby any day. The child is staying full time till Jan then going part time( to hold her spot) for the remainder of the mat leave when both children will be coming full time. I have budgeted for that income loss.