One cell seems to be bad. There are 6 of them. If one quits the job you'll get around 10 V.

Are you sure that the CTEC is connected correctly? I also use a CTEC and these are very good.

What is the charging voltage of the alternator? It should be around 14.5 V.

If all check out fine the only reason I can imagine is that someone said that even battery makers are saving costs and lead which leads to a shorter battery life. >5 years would be good.