18 Mbps does not equal 18 MB/s they are different measurements.
A Byte is eight bits. So a bit rate like 18Mbps = 2.25 MB/s. So your numbers are accurate.
You can blame marketing and tradition for using that metric. Modems were always in baud (bits per second) 9600 baud = 1200 Bytes/sec or 56Kbps = 7KB/s (though more like 5 KB/s in practice)
Since most internet connections actually do end up on the phone network they still use 64Kbps (56Kbps) as the smallest unit. ISDN connection was a pair of phone lines, T1 is a bundle of 24 line, and it goes up from there to T5. Also the Optical Carrier standard which is another step up. With the top OC-768 handling a staggering 40Gbps or 5GB/s