grep() slower than which(grepl())??

gistfile1.txt

library(microbenchmark)
N = 1e4
v = sample(letters, N, TRUE)

microbenchmark(times = 200L, grep("a", v), which(grepl("a", v)), grep("[a-m]", v), which(grepl("[a-m]", v)), grep("[a-z]", v), which(grepl("[a-z]", v)))
# Unit: microseconds
#                      expr     min       lq     mean   median       uq      max neval  cld
#              grep("a", v) 630.561 640.4110 670.5257 655.4915 680.6115  866.132   200  b  
#      which(grepl("a", v)) 598.802 609.5220 637.0542 621.0965 652.9120  839.231   200 a   
#          grep("[a-m]", v) 692.601 708.4215 735.6788 721.3065 744.6365 1207.722   200    d
#  which(grepl("[a-m]", v)) 652.362 671.8470 704.9352 687.3010 719.2070 1007.502   200   c 
#          grep("[a-z]", v) 639.452 649.2360 683.5104 661.7870 688.1360 1161.722   200  b  
#  which(grepl("[a-z]", v)) 589.322 598.3860 629.4264 607.7310 639.5370 1113.842   200 a  
 
Ditto for N=1e7