I may be wrong, but I think holding a slightly cross legged position does still require some muscle contraction, and it may cause some compression over having your legs relaxed and spread apart. Also, if you’re lying on your back, then there might be a slight pressure difference from having one leg higher than the rest of your body.

The way you’re describing it, these will all likely cause a slightly lower pressure.

It could also be that you were more relaxed during the second reading, and so that time was the most important contributor. For example, white coat hypertension

en.wikipedia.org/wiki/White_coat_hypertension