First two extract brews -- low potential ABV?

I've done two extract brews so far and I followed basically the exact same procedure both times. Here's a link to my previous "omg I'm new at this" post:

https://www.homebrewtalk.com/f39/racked-secondary-fermentation-early-134891/

I did a second brew last weekend and just racked it to secondary a few minutes ago. Here are the SG measurements I've taken so far:

Brew #1: Red ale
1.044 = SG after pitching yeast*
1.018 = SG when racking to secondary 2-3 days later
1.017 = SG when bottled, 13 days after pitching

*I previously said 1.046 in my other thread, but looking back at my notes, 1.042 was the hydrometer reading and 1.044 was the corrected reading since the temp of the wort was nearly 80 F.

Brew #2: IPA
1.047 = SG after pitching yeast
1.018 = SG when racking to secondary 8 days later

I may be totally wrong, but looking at the potential ABV scale on my hydrometer, the formula for SG-to-ABV is: %ABV_current = (SG_initial - SG_current) * 133

Using this formula, brew #1 is at 3.9% and brew #2 is at 3.7%.

I used the same yeast (Munton's dry ale yeast, with a starter) and the same technique on both brews, and I noticed that the SG of brew #1 dropped quickly by 0.026 over 2-3 days and then did not really drop any further given another 10 days. So it went from 0% ABV to 3.9% and then stopped. Then I look at brew #2, and it dropped by 0.029 over 8 days. Its fermentation activity (krausen and airlock bubbling) has matched the first brew. So I don't really expect its SG to drop much at this point. Both beers seem to taste and smell OK, so I don't think any contamination is going on.

What could cause me to not even reach 4% ABV when most people seem to pass 5% ABV easily? Was the starting SG just too low, too watered down?

There was/is nothing wrong with your beers and their gravities. An og between 1.040 and 1.050 is very common. 1.018 fg might be a little high for some styles, but well within the range of acceptable.