Apple Mac sales are slowing down by Chris Matthews @FortuneMagazine October 9, 2015, 8:08 AM EST E-mail Tweet Facebook Linkedin Share icons There’s trouble in Cupertino, Calif., as two research firms have estimated that sales of Apple AAPL Mac computers have been sluggish this quarter, according to a report in The Wall Street Journal. One of these estimates, from the research firm International Data Corp., said that sales of the computers actually fell by 3.4% year-over-year. Gartner was more optimistic, estimating that Apple increased sales by 1.5%. Apple shipped 5.6 million units, according to Gartner, while IDC’s estimate came in at 5.3 million units. Even with Gartner’s more optimistic view, these numbers would represent the slowest growth in Mac sales since 2013. “Mac sales may have slowed due to a strong U.S. dollar, which made the computers more expensive outside the U.S.,” the report said.