time difference between rows producing odd results - r

Question

Plenty of material on stackoverflow regarding calculating time differences between rows/entries/observations. However, I'm stumped why I'm getting NA's in unusual positions.
I have 3 columns, DATETIME which is posixlt, GRP800 which is the group (factor), and TIME800 which is supposed to represent the time elapsed between each observation for each group. My particular code was derived from Calculate differences between rows faster than a for loop?.
df$TIME800<-unlist(by(df$DATETIME,df$GRP800,function(x)c(NA,diff(x))))
It does appear to function properly for the first group but then I am getting NA's in the middle of the 2nd group. I've tried several approaches using diff and it's producing the identical output. I'm quite puzzled. Any advice would be greatly appreciated.
DATETIME GRP800 TIME800
1 2013-07-16 16:01:30 1 NA
2 2013-07-16 20:00:54 1 3.990000
3 2013-07-17 00:01:30 1 4.010000
4 2013-07-17 04:01:00 1 3.991667
5 2013-07-17 08:00:50 1 3.997222
6 2013-07-17 12:01:46 1 4.015556
7 2013-07-17 16:00:50 1 3.984444
8 2013-07-17 20:01:00 1 4.002778
9 2013-07-18 00:01:18 1 4.005000
10 2013-07-18 04:01:02 1 3.995556
11 2013-07-18 08:00:50 1 3.996667
12 2013-07-18 12:01:18 2 NA
13 2013-07-18 16:01:02 2 3.970833
14 2013-07-18 20:00:59 2 4.007500
15 2013-07-19 00:01:31 2 3.997222
16 2013-07-19 04:01:18 2 4.011111
17 2013-07-19 08:01:02 2 NA
18 2013-07-19 12:01:57 2 2.007500
19 2013-07-19 20:01:00 2 NA
20 2013-07-20 00:01:00 2 2.003333
> dput(df[1:20,])
structure(list(DATETIME = structure(list(sec = c(30, 54, 30,
0, 50, 46, 50, 0, 18, 2, 50, 18, 2, 59, 31, 18, 2, 57, 0, 0),
min = c(1L, 0L, 1L, 1L, 0L, 1L, 0L, 1L, 1L, 1L, 0L, 1L, 1L,
0L, 1L, 1L, 1L, 1L, 1L, 1L), hour = c(16L, 20L, 0L, 4L, 8L,
12L, 16L, 20L, 0L, 4L, 8L, 12L, 16L, 20L, 0L, 4L, 8L, 12L,
20L, 0L), mday = c(16L, 16L, 17L, 17L, 17L, 17L, 17L, 17L,
18L, 18L, 18L, 18L, 18L, 18L, 19L, 19L, 19L, 19L, 19L, 20L
), mon = c(6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L,
6L, 6L, 6L, 6L, 6L, 6L, 6L, 6L), year = c(113L, 113L, 113L,
113L, 113L, 113L, 113L, 113L, 113L, 113L, 113L, 113L, 113L,
113L, 113L, 113L, 113L, 113L, 113L, 113L), wday = c(2L, 2L,
3L, 3L, 3L, 3L, 3L, 3L, 4L, 4L, 4L, 4L, 4L, 4L, 5L, 5L, 5L,
5L, 5L, 6L), yday = c(196L, 196L, 197L, 197L, 197L, 197L,
197L, 197L, 198L, 198L, 198L, 198L, 198L, 198L, 199L, 199L,
199L, 199L, 199L, 200L), isdst = c(1L, 1L, 1L, 1L, 1L, 1L,
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L),
zone = c("MDT", "MDT", "MDT", "MDT", "MDT", "MDT", "MDT",
"MDT", "MDT", "MDT", "MDT", "MDT", "MDT", "MDT", "MDT", "MDT",
"MDT", "MDT", "MDT", "MDT"), gmtoff = c(NA_integer_, NA_integer_,
NA_integer_, NA_integer_, NA_integer_, NA_integer_, NA_integer_,
NA_integer_, NA_integer_, NA_integer_, NA_integer_, NA_integer_,
NA_integer_, NA_integer_, NA_integer_, NA_integer_, NA_integer_,
NA_integer_, NA_integer_, NA_integer_)), .Names = c("sec",
"min", "hour", "mday", "mon", "year", "wday", "yday", "isdst",
"zone", "gmtoff"), class = c("POSIXlt", "POSIXt")), GRP800 = c(1L,
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 2L, 2L,
2L, 2L, 2L), TIME800 = c(NA, 3.99, 4.01, 3.991666667, 3.997222222,
4.015555556, 3.984444444, 4.002777778, 4.005, 3.995555556, 3.996666667,
NA, 3.970833333, 4.0075, 3.997222222, 4.011111111, NA, 2.0075,
NA, 2.003333333)), .Names = c("DATETIME", "GRP800", "TIME800"
), row.names = c(NA, 20L), class = "data.frame")