Is there any reason why the 'y' in 'sytti' sounds different from normal?
It's seems like you've made the mistake of thinking the spelling of a word reflects the pronunciation of it. This is only partly true, but there are a lot of of exceptions. In addition to this, there are many regional variations in the pronunciation of words. In this case both 'søtti' and 'sytti' are common ways of pronouncing '70'.