You are confusing your units.
M = mega
m = milli
B = byte
b = bit
When referring to disk usage, we measure throughput in megabytes per second, or MB/s. Notice the capital M for mega and the capital B for bytes.
When referring to network performance, we measure throughput in megabits per second, or Mb/s. Notice the lowercase b.
A bit is eight times smaller than a byte. You can figure out your 100Mb/s network's maximum theoretical throughput in MB simply by dividing by 8.
100 / 8 = 12.5.
TCP/IP has ~ a 10% overhead, as does Ethernet, so realistically you'll only see about 80% of that at the high end. A little more basic math shows that
12.5 * .8 = 10. You should expect to be able to write at about 10MB/s over your 100Mb/s network. This lines up perfectly with what you are seeing.
tl;dr - Capitalization is important.