Why condiments expiry date is rounded?

Food quality isn't binary; it doesn't go from perfectly good to perfectly bad in an instant. Even if it did, the time it would take depends on the storage temperature. And for non-liquid foods, it's possible that only a part went bad (how well does it mix?).

So, you don't get a precise date, but a rough interval at which time the decay starts to set in. As a result, the manufacturer will just pick a rounded date from that interval.

Example: the engineers might calculate that under reasonable circumstances, the product may start to noticably deteriorate after 52-75 days, and become dangerous after 81-112 days. They manufacturer could then say that the expiry date would be 60 days.

(The other answers explain why you'd use the first interval, but not why they're actually intervals.)

:edit:

The likely reason why they're all the same 60 days is probably also engineering. How much preservative do they use? As noted in the comments, the primary preservatives are the acids, but you need quite some sugar to compensate. It seems 60 days is a commonly accepted balance.

I'm not sure what the consumer liability laws are like in Brazil, but I expect that they 'good until' dates well short of their 'actual' lifespan to protect the manufacturers from lawsuits for people who don't read the labels anyway and get sick on 3 year old mayo...

A second reason they set short shelf life for such things is to encourage you to buy it more often, thus more money for the manufacturer.

Expiration dates for safety are not about "usually okay". Sure, if you keep mustard in good conditions, it'll probably last longer than that most of the time. But things are labeled with expiration dates that are designed to guarantee that everyone will be safe. Yes, that means that most of the time they're overly conservative. But the alternative is to let people get sick, or even die. It's a big world; if even one in a million bottles goes bad after 60 days, that's a big deal.

If by "round" you mean 60 days as opposed to say 58 for some things, and 63 for others, it has to do with the perceived accuracy of numbers. If you read "keeps for 53 days after opening" you might feel it has gone bad after 54 days. But with 60, it's a round number, you realize that there's essentially no different betweeen 59 day old ketchup and 61 day old ketchup. When you find a 90-day-old bottle at the back of the cupboard, maybe you decide you'll toss it because it's so far past 60 days. If you're wondering why so many products have exactly the same shelf life, I think the manufacturers tweak the formula - who would buy a jar of mustard that went bad in just few days? It needs to last about as long as it takes people to use up the jar or bottle.

Me, I keep my ketchup in the cupboard (in Canada, the large bottles say refrigerate after opening and the small ones don't) and keep it for months and months - maybe even a year. No worries. But my chef-daughter announces that a product (eg a carton of milk) "goes bad" on it's expiry date, and refuses to consume anything on or after its date.

The manufacturer has to test that the expiration date is valid and each tested point has some cost associated with it. Lets say they test with 100 containers at 60 days, testing an additional 100 containers at 61 days has significant incremental cost but very little incremental value.