A few things will do this.
Lower mash temp. Calibrate your mash temp gauge. You may be unintentionally favoring beta-amylase in your mash. Especially if you mash within a couple degrees of 149°F.
Holding temp mash better in warmer conditions you may be holding temp better and converting more starches. If you mash around 152° but normally while cooler your mash drops below the ideal range for alpha-amylase, and fall into beta ranges you would have a less fermentable wort in your old batches. Because the beta-amylase would have been denatured at the higher temp. You should do an iodine test to make sure saccrafication is done, rather than relying on a timer.
Lager yeast contamination your brewhouse may be haboring some lager yeast. Lager yeast will consume maltotriose drying out beers more than an ale strain.
PH change of you use municipal water the PH may have changed from your norm. Making better conditions in the mash and getting more enzyme action.
Warmer fermentation will attenuate better even if primary temp is the same as the past but allowed to be warmer in final stages.
Wild or Brett contamination these strains will even eat sugars from wood. But are pretty easy to identify as they will change the beer flavor significantly, but worth mentioning.
Hydrometer failure you should test the meter using distilled water. It should read 1.000.