If they had anticipated competent rock at say 15m depth then that is where the concrete piles design would go to. But if the competent rock isn't until 50m depth, of which there's a good chance as the Sherwood Sandstone can be extremely week for dozens of metres in depth, then the piling costs could be many many more times than first thought.. With the proximity of the river I would have thought that the rock would be quite weak for quite a depth tbh.
Only Everton, really wouldn't surprise me at all. In fact it wouldn't surprise me if it were a bottomless pit.