According to the Deutscher Wetterdienst press release the average for Hamburg in June was 17,6°C while the long term June average is only 15,7°C. June is often still fairly cool at times (like "Schafskälte" periods). Actually the reference period from 1961-1990, compared to the more recent and warmer reference period from 1980-2010 it was only about one degree warmer according to their press release. I think it just felt colder because of all the rain, so there were less sunshine hours than average and it didn't get noticeably warm towards the end of June.