Only north south blocks start at 0 at City Hall. Market St. is the demarcation for north south blocks. But Broad St. is not the dividing line for east west, Front St is.

So one block north or south of Market is 100, and one block east or west of Front St is 100. Broad st is the 1400 block because it is 14 main blocks west of Front. You only really run into east addresses once you get into Fishtown and Port Richmond because people actually live on the other side of Front St.

What's more is that you only have to address mail east or west if there is a corresponding address. You wouldn't say 200 West Market St because there is no East Market St. But you do have to specify between 200 E or 200 W Girard.