Maybe they watched it later on video to check if it was right?
Probably the chair umpire forgot one...
Same with Karlovic in 2005:
"The chair umpire's scorecard for the first-round match showed Karlovic had 55 aces against Daniele Bracciali.
Postmatch statistics credited Karlovic with 51. Tournament statisticians reviewed a tape of the match and concluded that total was accurate, Wimbledon spokesman Johnny Perkins said." Source