Why does the radio's meter show less power output than my external watt meter?

That's an excellent question. In general, the meter on the front of the radio isn't nearly as precise as an external watch meter and it's not unusual to see less power output on the radio's meter than on a precision meter like a Bird 43P or 4391 Digital RF Analyst.  What the meter on the radio will do is give you a general idea of the power output, and then if you notice a sudden drop, for example, it allows you to know that something unusual is occurring and then you can double check it with an external meter