If using fashion photography as a reference (which the image on the left looks like), the answer is yes it's a trick of perspective. It's a norm in fashion photography to shoot from a slightly lower angle, tilted a bit up at your subject. Like Patrick mentioned about fashion drawing, it adds length to the legs, reads as elegant and is flattering to models.

I use the top of the leg bone, at the great trochanter as my general line of middle. Of course this changes from person to person, but I find it is a good starting point. It is a little higher than the bottom of the crotch, and even for these two poses fits a bit better. Cheers!

It varies widely person to person but for a quick average it tends to fall about middle. In your first reference, consider the platform footwear elongating the legs, and that she is leaning back against the railing slightly which puts the leg into perspective, so it's tough to exactly find the middle, but with quick measurements, I see it pretty close to mid-way of the body. In the second reference, his legs are splayed apart in perspective, which lowers the crotch slightly in the overall measurements of the body, but still I see it as just about middle. When it comes to quick sketch gestures, using the crotch has a halfway point is just an easy way to keep your proportions in check, but it will likely vary depending on the model, perspective, or what you're trying to do with your figure drawing. Michael Hampton uses the crotch as a mid point, and suggests that if you need to adjust proportions to add height, it's best to add more length to the legs, which is often done in fashion and will read as a more graceful, elegant figure. Hope this helps!