Confusion regarding a 12V DC motor's RPM
DC motor RPM is highly dependent on load. Motors sold for industrial usually have specifications that provide RPM at rated load. Motors sold for hobby use often state the no-load RPM. Sellers of motors for hobby use often don't pay much attention to accuracy, clarity and completeness of specifications. I suspect that the problem is not so much that manufacturer's lie but that some manufacturers sell junk.

A gear that reduces the motor speed from 10,000 RPM to 100 RPM represents a significant percentage of the motor's load. Differences an the quality and age of the gears may have a significant impact on the output speed of a 45 watt motor.