Is this Realtek wireless adapter really a 300Mbps device?

I bought a cheap, unbranded wireless adapter off the net, the chipset for which is REALTEK 8191.

The specification states - Data Rate: 802.11n up to 300Mbps (downlink) and 150Mbps (uplink).

Whenever I look at the status of the device, it either reports a speed of (more often than not) 135Mbps, or occasionally 150Mbps; yet a Netgear WNA3100 adapter that I have can report speeds of up to ~280Mbps when connected.

Is it likely that this Realtek adapter isn't really a 300Mbps device?

Thanks for your thoughts.

Manufacturers can play a lot of games with performance specifications. They specify 300Mbps but that would be achievable only under ideal conditions not normally seen in the real world. And as this is a cheap unit the specification may be highly optimistic.