Nope you are transplanting 3E/4E expectations into 5E. Level has nothing to do with 5E encounter building guidelines. It's xp but a CR 15 critter is not for lvl 13 to 17.I suppose, but you're expected to die in those cases, in which case your accuracy isn't super relevant. Anyone can trip over the Tarrasque, regardless of your level or the edition you're playing.
The meta-game guidelines for 4E are that you should encounter monsters that are within two levels of your own (IIRC). The meta-game guidelines for 5E are that you should encounter a certain XP budget worth of monsters and that none of those monsters should be higher level than the PCs. You're free to ignore the guidelines, but whether the encounter turns lethal because a single monster is higher level, or whether it's because the XP budget for that fight is too high, both cases are because you've ignored the guidelines. The XP budget guidelines aren't somehow more canon than the level-limit guidelines. They're all just suggestions.
Even a "deadly" encounter in 5E isn't that hard. For example assuming a 5 person party. The budget for a deadly encounter is 5500 xp CR 9 is 5000xp CR 10 is 5900.
At level 10 the budget is 14000 xp. CR 15 is 13000 xp CR 16 is 15000.
Level 15 is 32000xp CR 20 is 2500p CR 21 is 33000.
Level 20. Xp budget 63500 xp. That's roughly CR 24.
Things also don't really get deadly as such until you go triple to X5 over the xp caps. If you do exceed it reduce the number of encounters per day. CR 25+ is for high level play where you only have a few encounters so nova away.
Personally I think the guidelines are trash but I share the same opinion of the 3E and 4E ones. CR 13 can be fine on level 7 PCs and it's only a few points over the xp cap.
The encounter guidelines are basically gamist all editions. The old wandering monsters tables from AD&D are more for living world type games. Just because PCs are level 4 they can still encounter old dragons. Suggestion be very polite and persuasive.
Much like the real world don't go to certain parts of the world as a civilian.