It isn't. It gets even worse if you get a 3rd party spool to replace the original. If the new spool doesn't have the exact same specs as the original, the IPT will be different than the rated one.
Gear ratio will tell you how fast the spool spins on retrieve. This remains constant no matter the spec of the spool or how much line is on the spool.
IPT is constantly changing - not knowing at what point it was measured by the manufacturer, you have no idea what the actual IPT is at various spool-fill points.
If the spool is 1/2" deep, then the difference in IPT between an empty spool and a full spool is 3.14" per turn.