Thanks for running a relay. (:
Clients choose which relays to use based on their consensus weight. The consensus weight is assigned by the directory authorities after measuring the throughput for your relay.
There are a number of things that can impact the speed at which your relay can be used, including the CPU speed, CPU architecture, the network interface and the network environment around you.
It may be that you are just unlucky and the bandwidth authorities have a poor connection to you, while you may have a really good connection to other sites.
The bandwidth authorities will also group you with similarly rated relays when they perform the measurement, this means that your maximum measured speed is limited by the slowest relay in the group. You may have had a poor measurement when first measured, and are now limited by the group you've been placed in.
I wouldn't worry about the speed too much. I see currently it's 3.17MiB/s (which is 25.36Mbps), which is a very respectable speed for a relay.