surely at 13m I should be off-gassing enough to allow me more than 1 minute at that depth?
To simplify, forget about off-gassing for a moment. Picture on a shelf, two buckets with heights 30 cm and 20 cm, representing the NDL surfacing limit. (If they overflow, you've gone into deco.) These are connected to one of several water pumps on the floor. The more water in the bucket, the harder it is to pump more water in, so the amount of water you can put in the bucket depends on the strength of the connected pump. The other difference between the buckets is they are being filled through a hole in the bottom by hoses of different diameter -- the 30 cm by a large hose, the 20 by a small hose. Since the pump output (pressure or water forcing power) is the same, the skinny hose (shorter bucket) fills slower. Lastly, someone turned on the pump before you got there, so there are different amounts of water in each bucket.
Now consider the pump output is related to the ambient pressure at 13 m. Unfortunately, that's strong enough to overflow both buckets. The water level and fill rates are such that the large bucket will overflow in 3 minutes and the smaller bucket in 5 minutes. NDL is therefore tied to the large bucket, giving you 3 minutes.
You notice another pump next to the 13m pump labeled 12m. That's a weaker pump, so you disconnect the 13m and connect the 12m pump to the hoses. (This is the analog to ascending from 13m to 12m.) Conveniently, the large bucket can hold more than the weaker 12 m pump can output -- it will never overflow! Cool, we don't have to worry about that bucket any longer. Unfortunately, the slower-filing bucket is not tall enough to hold enough water to stop the pump. The current water level and ingress speed are such that it will overflow in 10 minutes. Your NDL therefore jumps from 3 minutes to perhaps 10 minutes since the controlling bucket switched. This is how a large jump in NDL can occur.
If you connect the 11m pump, the slower flow rate will overflow the small bucket in perhaps 12 minutes. (Also understand that as the level in the bucket approaches pump capacity, the rate slows even further for a given pump.) Connect the 10m pump and it will overflow in perhaps 20 minutes. Connect the 6m pump -- which is not strong enough to ever fill either bucket -- and you can stay there all day if you had enough gas.
Now, one point that perhaps is confusing you:
Off-gassing is not a concern for the NDL time shown. By definition, NDL means you can directly surface at any time. None of your buckets are filled to the rim -- or past, if you can imagine such a thing. (FWIW, your Suunto Ocean uses 16 buckets.) As you connect weaker and weaker pumps (i.e., ascend), the level in the bucket will become higher than the pump capacity, so the water backflows & the level drops (aka off-gassing). We don't care about that for NDL because we had already stopped worrying about that bucket. It is an important consideration, though, as less water in the bucket means more margin from overflow, as well as contributing to the setup for your next dive. If you bump a nearly full bucket (i.e., strain while climbing the boat ladder or schlepping tanks to the car), you're more likely to overflow (i.e., DCS hit) compared to a less full bucket.
To wrap up the analogy: the buckets are tissue compartments in your dive computer. The bucket height is the desired surfacing limit (in your case 90% of where Buhlmann wanted to draw the line). The pump strength is the inert gas partial pressure at whatever depth you're at. This partial pressure is the ambient pressure (2.3 bar at 13 m) times the nitrogen fraction (0.79 for breathing air, but less for nitrox). In other words, diving nitrox weakens the pumps so it takes longer to fill the buckets (their size doesn't change) leading to longer NDL times or more margin (or a mix of both).
