This information comes from Petri's Brad Sams, who spoke with several sources at the companies in question and tested the theory for himself. At least some OEM devices are shipping with Windows 10 1803.
The issue, according to sources, is that there wasn't enough time to test the hardware of these devices with 1803. They were always planned to line up with Microsoft's release schedule, shipping with 1809, the October 2018 update.
As a result, the bulk of the testing would have been done on pre-release versions of 1809. Shipping on a different build could result in potential compatibility issues, negatively affecting their customers.
Windows 10 1803 and ARM
But the headache gets worse. Manufacturers have been marketing devices with the features of the October 2018 update, but have been unable to deliver on them. Microsoft hasn't given consumers an estimate of release, and allegedly it hasn't told OEMs, either.
Sams gives Lenovo's YOGA C630 as an example. There are units in BestBuy right now on 1803 when it's not designed to run it. As one of the new Always Connected ARM-based PCs, the laptop holds the Snapdragon 850. A processor Microsoft's support documentation lists as supported only on Windows 10 1809.
If it carries on, Microsoft could very well alienate its OEM partners. It's far from the first time a Windows update hasn't shipped on time. When they do, they often hold bugs anyway, which again affects the satisfaction of those companies' consumers.
It's not common for devices shipping older builds to run into too many issues, but it'll be interesting to see if any surface. A new version of 1809 is doing its run on the preview ring, so hopefully OEMs and consumers won't have to wait much longer.