Knowing your temps are inaccurate it's really hard to say. The drop could have been because of a change in time or it could be a legitimate drop that would have happened either way. Normally I wouldn't recommend this, but you may want to try using a
Temperature Corrector to set your temps to a matching time. Then make sure you continue to actually temp at that time each day so your chart will be more accurate. Just be sure to make a note of what each temp was originally. It still wouldn't be 100% accurate, but it would paint a much clearer picture.