How come string and pattern monitors are implemented differently from other types of monitors?

A: On-change monitors

While the Monitorpattern IDL interface may be defined in exactly the same fashion as other monitor types, it is actually quite different. The most important aspect here is the delta value passed to the set_value_trigger method is ignored entirely by the C++ implementation of this monitor interface. You see the concept of a delta change value does not really apply to a fixed array of 64 boolean values (which is identical in principle to a pattern property). The way this monitor really operates is that any change from one period of time to the next in the property's value triggers the monitor.

A: Monitorstring IDL interface is defined but string properties return monitors of type Monitor

string properties return monitors of type Monitor instead of the Monitorstring IDL interface because the monitor subinterfaces' only purpose is to deal with on-change monitor properties. With a string, it is impossible to change a delta value so there is no reason to create the subclass (aside from consistency).