Because they also help run other events too, perhaps?? Sip Events have been in Plymouth for most of last week and the start of this week helping out at Twentyfour/12.
Also they do warn you that they aren't sitting in offices all day everyday:
From the 'contact us' section of the website
(We usually only answer emails during office hours Tues-Thurs. We try to answer emails within 2-5 days, but it may take much longer during the busiest periods of the race season.)