Unexpected Video resolution in Mediastreamvalidator

I noticed that the Video resolution reported by mediastreamvalidator does not always match what is in the NAL for a stream. For example I have an HLS stream with 4 channels; one is 352x480 but mediastreamvalidator reports it as 853x480. I have checked the SPS NAL and it has the width in mbs as 21 (22x16=352) as expected. Does anyone know how the mediastreamvalidator determines the frame size?


Thanks,

Phil