Ya know, I have no answer for that and often wondered myself about the whole sell-by date conundrum. Even more confusing than the julian date (how many ppl carry a calendar with them that has the days of the year sequentially numbered for easy reference?) are the encoded dates. As a former assistant manager for Walgreens, I was tasked with checking of ALL pull dates in the store! This was a horrendous project (think of all the bottles of pills to start with... then there's baby formula... canned goods...candy...) made even more upsetting by the fact that some of the more perishable items had dates that can't be read without the use of a code book! I personally think it should be illegal to use a sell-by date that can't be read by the comsumer! What's the point, other than driving the poor employee insane? To me, it almost smacks of trying to sell goods past their date because the biggest watch-dogs out there are the consumers, bringing out-of-date product to the employees' attention.
And don't even get me started on cheese... as a former Deli Manager for 12 years, I had to throw away perfectly good cheese from my cheese table because it had reached its sell-by. A nice hunk of grana padano, vac pack still sealed and NO MOLD had to go into the garbage. We weren't even allowed to donate it to the local food bank because of liability issues...
OK, I wasn't going to get started.
Then, there's the whole issue of proper handling from the day that the sell-by was applied. If, during anytime in shipment or at store level, the cheese was allowed to warm above ~45 F for more than about two hours, the shelf life is drastically shortened and the consumer would have no way to know other than the obviously bulging package.
So, bottom line is, I'm guessing it's a julian date, because that's the shorter code of the two and fresh mozz is highly perishable, but you could also be right... it's an error with the imprinting that got through any quality control point there might be. However, that date becomes useless as soon as you open the container. Then, the next clock starts ticking and you have only a matter of days to use it up before it goes over. And, after all this, I'm sorry to hear that the cheese wasn't that great because I'm sure that it was quite dear, being imported.