D&D 5th Edition: Truesight and Darkvision, Why Does A Monster Have Both?

While creating a homebrew monster based around eyes and vision, I looked up monsters that had both darkvison and truesight, surprisingly only two have both, the Avatar of Death and Canoloth, I’ll use the Canoloth as the example here.

When reading the descriptions of both vision types, darkvision allows a creature to see in dim light as if it were bright light and darkness as if it were dim light but it can’t discern color and only sees shades of grey, with truesight not only can you see in normal darkness but also magical darkness, as well as many other benefits, so what confuses me is why any creature would have both forms of vision (especially when it only has darkvision out to 60 feet but truesight out to 120 feet) when truesight already has the only benefit of darkvision along with all its other benefits?

Have I misinterpreted the mechanics of these different sight types, is their a hidden reason behind having both? Or is it just a slipup of the designers to give a creature like the Canoloth both forms of vision?