You're withing the margin of error. The new SO-DIMMS (memory) may have slightly different timings too which can move the score. But...it's essentially the same. Don't even sweat it. It's better to have MORE RAM even if slightly lower latency or even lower clock speeds than to be starved for memory and caching everything out to the hard drive (2013). If you're really looking for an upgrade, put a SATA SSD in there but mind the stupid fans going into high speed because of the whacky way Apple builds its iMacs.