If you have an AAD and think that you are at or below 1,500 feet in freefall, you should just pull the reserve because there is a chance the AAD will fire whether or not the main is deployed. This is not a design problem in AADs and is probably true of all models. It is a fact of physics that if you look at your altimeter at 1,500 feet, you will be at 1,000 in about 2 and a half seconds. It takes that long just to open the container. It would be another one or two hundred feet before the main canopy opened.
Instability can add to the problem by causing an erroneous altitude reading.
Finally, many people who guess their altitude after the fact may be subconsciously deceiving themselves and estimating on the high side. After filming a 4-way practice jump, the cameraman continued in freefall and deployed at the same altitude as one of the team members. Each had an off heading opening of about 90 degrees, leaving them facing directly at one another. Both were under Sabre 135s at about 1.2 wing loadings, giving them a closing speed of perhaps 30 to 35 miles per hour. About three seconds elapsed from inflation to collision, indicating that the two opened with about 150 to 200 horizontal feet of separation.