Because seven ate (eight) nine.

Source: http://tinyurl.com/pa2xcta
English speakers are increasingly punctuating their speech with ‘um’ rather than ‘er’, according to socio-linguists at Edinburgh University. So why the shift?
In the historic struggle between the ummers and the errers, the ummers are getting the upper hand. A study of speech patterns by socio-linguists at Edinburgh University has found that English speakers increasingly tend to use “um” rather than “er” as the filler of choice.
Prepositions, participles, who and whom and more are the focus of the new documentary “Grammar Revolution.” The film was made by two former teachers, David and Elizabeth O’Brien, and completed with a $22,000 Kickstarter campaign. It’s a feast for grammar geeks.