The arithmetic would be too much for a large number of tillers, who can't do much more math than count change. This still misses the other main purpose of the

The arithmetic would be too much for a large number of tillers, who can't do much more math than count change. This still misses the other main purpose of the .99p, which was to force the tiller to open the till to get the change (thus, reducing the likelihood that they might just pocket the money handed to them).