First - More information. Did you do a single point test or a multi-point test with the blower door? Did you calculate any leakage ratios? LBL? Canadian Eqa-l? Where did you find the leakage coming from? Where did you not find leakage coming from? Did you do any IR with your blower door, what does that tell you?
One of my tightest houses was a 1912 one story bungalow. About 30 percent of the leakage was the attic hatch. The rest was weatherstripping of doors, windows, and the minimal amounts coming through wall cavities. There is no insulation in this house. So blowing cellulose in the walls and adding cellulose to the ceiling, will provide some air sealing as well. This house has had only one minor addition in the century of use. A back porch was enclosed.
I believe these houses have a firm foundation, they have some quality craftsmenhip in parts of the house. The lathe and plaster is not cracked.
Those things all help.
Please forgive me, but I am simply going to bypass the numbers and talk about an issue I do see in some of these discussions and hopefully answer your question at the same time. First, contrary to popular belief, a blower door test does not test the entire house - just the envelope. This can be a real issue if one finds a super tight house and assumes everything is fine and doesn't check for air leakage between the basement / crawl space in the houses as you can have not only thermal losses but a fire & safety issue.
In answer to your question, John touched on a few of the possible causes for having numbers lower than you were expecting but herein lies my list. Hiring a real painter that does his job properly will cut down on air leakage. Older homes with numerous layers of flooring, walls, wall paper, paint, etc... will also have lower than anticipated leakage rates as they help block infiltration (I have been in numerous houses with 3 or more layers of flooring, horsehair plaster with wall paper, covered over with drywall & / or paneling, etc... I am sure you can imagine how low some of those test). How about a homeowner that follows articles found on my website or others on DIY air sealing - or possibly retrofitted or replaced their older windows and filled in the sash pockets. As a quick reminder, most insulation used does not act as an air barrier, nor should it be treated as one (exceptions to acting as an air barrier is CC Foam & Dense Pack - still should not treat it as the primary air barrier though)
One more question for you (besides the ones John asked) on the gable vents - was this the only ventilation, or was it replaced properly with soffit & ridge vents? (I would ask about a hot roof setup, but I assume by no insulation - that means none anywhere)
John and Sean both do a good job of trying to answer your question of why the house isn't leaky. I'll add a couple of points. Your questions really makes it sound like you're very surprised the house wasn't leakier, but I'm not clear why. Just the age of the house?
Sean's points about insulation is very true. The presence, or lack of, insulation is not related to the how leaky the house is. Most insulation provides only a thermal barrier.
Regarding the attic and any crawlspaces. It's very important that those areas be open to the outdoors. If the attic is completely sealed up, then it would probably have a pressure somewhere between that of the house and the outdoors. This will artificially lower the measured leakage