I found a technical paper on asymmetric gear teeth design that does a good job explaining their purpose in the introduction:
"In propulsion gear transmissions the tooth load on one flank is significantly higher and is applied for longer periods of time than for the opposite one. An asymmetric tooth shape reflects this functional difference. Design intent of asymmetric gear teeth is to improve performance of the primary drive profiles at the expense of the performance for the opposite coast profiles. The coast profiles are unloaded or lightly loaded during a relatively short work period. Asymmetric tooth profiles also make it possible to simultaneously increase the contact ratio and operating pressure angle beyond the conventional gears’ limits. The main advantage of asymmetric gears is contact stress reduction on the drive flanks, resulting in higher torque density (load capacity per gear size)."
DIRECT DESIGN OF ASYMMETRIC GEARS: APPROACH AND APPLICATION*Alex Kapelevich, AKGears, LLC, 316 Oakwood Drive, Shoreview, MN 55126, USA
Based upon my understanding of the above, I would guess they used 22 asymmetric teeth to increase the load capacity for the gears during drive, but kept 8 teeth symmetric to maintain an acceptable level of performance during coast. If that's the case I think the asymmetrics are a better gear, but also probably more expensive to manufacture (which may be why they switched to symmetric gears at a later date).
Joel