something odd with WPA

I compared the diff of two plays' home_wp_post and WPA in the database.
Is WPA suppose to be the diff of two plays' home_wp_post?
Most numbers check out, but some numbers dont make sense.

Why timeOUT has a different home_wp_post?

Here is what i do:

tic()
test<-pbp %>%
         filter(game_id == "2009_18_GB_ARI",!is.na(home_wp_post)) %>%
         select(game_id,play_id, qtr, desc, total, spread_line, home_wp_post, wpa) %>%
         collect()
toc()

tic()
test <- test %>%
     mutate(wp_diff1 = abs(wpa))
toc()

tic()
test[1,'wp_diff2'] = 0

rownum <- nrow(test)

for (i in 2:rownum){
test[i,'wp_diff2']=abs(test[i,'home_wp_post']-test[i-1,'home_wp_post'])
}
toc()

temp<-test%>%filter(wp_diff2!=wp_diff1)

Start by reproducing the supplied R query and comparison against the database fields home_wp_post and wpa for game 2009_18_GB_ARI. Review how WPA and home_wp_post are defined, especially for timeouts, then document whether the values should differ and identify the cause of any remaining mismatch.