Delays can be anything from production mistakes to shipping issues. Usually at the factories they like to be involved in supplying everything including the hardware from my experience. They might be adding the instruction manual here after a quick QC check, could be a printing delay or correction. We used to check all our products and make sample inspections. Sometimes we had to add something to the product box even if it was just an inspection seal before we shipped product.
Shipping by sea takes 20 days but customs can be difficult and especially if they combined shipping in the container.

So who knows what the delays are, it’s not unusual, but to save face the company should be keeping the info coming and provide realistic dates.