Notice 2 of the numbers
have alternate readings (4 and 7). The alternate readings are
generally used for Dates and Time. But not necessarilly 100% of
the time. When counting floors in a building, Yon is generally
used because SHI is a Homonym for "Death" and that would
be unlucky in an elevator. The month of April, however is "Shi-Gatsu".
Listening to a native is really the only way to figure out when
to use which reading when.