When is 4:2:2 not 4:2:0

4:2:2 is generically interpreted to be 4 samples luma, 2 chroma cr, 2 chroma cb but ignores the field based nature of most video capture

4:2:0 is a somewhat interpretive acknowledgement of the loss between fields interpostively

4:2 is Luma:Chroma over all where ::0 is the alternate state or field where no chroma subsampling takes place and the previous chroma subsampling for the associated field is substituted for the same or similar pixels which are going to take the place of a chroma sample for both fields combined.

 You can think of this as loosing information when the two fields are "smushed" together, or taking advantage of the similar or identical nature of the chroma sample and removing the redundancy.

Some have interpreted this as "skipping" a chroma sample inter field, or only taking a chroma sample every other field on the odd or even field.

This can be true or partially true for MPEG2 in certain cases, unless fields are retained, in which case it returns to 4:2:2 as each field much have its own chroma subsampling field to remain a part of the video stream.. but again this may require a software codec to deal with the extra information.

Whether any hardware codec would expose fields or retain their 4:2:2 format remains to be seen.