Here's a comparison of the meter's serial protocol and that provided by the adapter. The meter continuously sends packets every 250 ms:

In a system with un-flushable buffers (USB-to-serial adapters), an application can not be guaranteed of obtaining the most recent value. Even worse, it cannot be guaranteed of obtaining a value that was measured after the read request was made. This makes automatic measurement sequencing impossible.

In contrast, here's the protocol implemented by the adapter:

Upon receiving a read request, the meter flushes the small local receive buffer, then waits a period of time to ensure the sample returned was taken after the request was made. This ensures synchronization.