In a rotary engine, displacement is a bit trickier. Each rotor has three faces, or working chambers, and there are two rotors in a 13B engine. All six chambers have a displacement of 0.654 liters, but it takes three revolutions of the output shaft to bring all those chambers through a full cycle. Were it to be rated by the same method as a piston engine (chamber displacement times number of chambers), it would be called a 3.9 liter, which is hardly logical. The accepted approach is to treat each rotor housing as a cylinder and measure the displacement of one chamber per rotation. Interestingly, this happens to be precisely equal to capacity. In other words, on every revolution of the output shaft, the rotary engine moves a volume equal to its rated total displacement.
For the purpose of displacement equivalency, either the rotary engine total displacement needs to be doubled or the piston engine total displacement needs to be halved. The latter approach is more correct (equates with capacity) but the former is simply more politically correct, as this is a piston engine world and those exaggerated ratings are just too familiar to our culture. So, rather than say that a 2.6-liter piston engine has 1.3 liters of capacity (true), we say that a 1.3 liter rotary has equivalent displacement of 2.6 liters (also true, but back-assward).