Hi Susan, there is a bit of confusion between metric system and "inch system" ( How is this called correctly in english ?) as the numbers don't fit exactly.
10cm are around 4 inch. So you have to divide our Ends per 10 cm by 4 to get the EPI. This works fine for 60/10 = 15Epi and 40/10 = 10EPI
Our 30/10 corresponds 7,5EPI and is mostly treated as 8EPI.
If you count 10/inch you must have got a 40/10 = 10EPI instead of the ordered 30/10