完全なコマンドパスを指定せずに詳細モードでtimeコマンドを使用できないのはなぜですか？

与えられた...

~$ which time
time is a shell keyword
time is /usr/bin/time

これがうまくいくのはなぜですか？

~$ /usr/bin/time --verbose ./some_script.sh 
Command being timed: "./some_script.sh"
User time (seconds): 0.00
System time (seconds): 0.01
Percent of CPU this job got: 51%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.03
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 4572
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 1
Minor (reclaiming a frame) page faults: 1548
Voluntary context switches: 3
Involuntary context switches: 7
Swaps: 0
File system inputs: 48
File system outputs: 0
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0

なぜできないのですか？

~$ time --verbose ./some_script.sh 
--verbose: no se encontró la orden

real    0m0.124s
user    0m0.101s
sys 0m0.022s