How many channels do ultrasound images have?
What is happening has nothing to do with the type of image (ultrasound in this case). It has to do with the type of file (JPEG in this case). By default, cv.imread assumes it is a 3-channel color image. You can specify grayscale when calling cv.imread if appropriate. The last dimension (the color dimension) in the im object could be empty.

There is also depth info (8 bits per channel). The default is 8 bit, you could specify 16 bit or 32 bit if apporiate.

Do not rely on function or software defaults. It is better to work from first principles of the raw data.