/usr/bin/time: not the command you think you know
The reason 'time' is a builtin is because it can time a complete shell pipeline, not just a single command. If you type    time foo |bar

Then the result is the total time taken by foo and bar together. This requires it to have special-case syntax. Whereas    /usr/bin/time foo |bar

Would run foo and give its time statistics as input to bar.