OpenJDK is just source code. Binaries (built by various companies and orgs) comes from someplace else, and are usually called something else. E.g. RedHat calls theirs "IcedTea". Azul calls their binary distro of OpenJDK "Zulu".
When you get a binary build of OpenJDK from some place (in tar/zip form, or deb or rpm package form), some of those are well built and tested (e.g. Azul's Zulu and RedHat's IcedTead, and obviously Oracle's own JDKs). But some may not be. they may just be a random build from a random point in the source tree, using unknown tool chains and shipped with little or notesting. E.g. the Java 8 version you'd currently get on the experimental debian:sid repos is base don 8u40: a source tree that is still in flux and not planned to be released as 8u40 until March of 2015.
Many people care about running on a JDK version that was actually tested and verified. Right now Zulu is the only binary build of a Java 8 OpenJDK that meets that need. And on MacOS or Windows, it's the only one for 7 and 6 as well.