2 way lighting domestic

I'm a time server qualified electrician but have only every worked on industrial installation. So not very familiar with domestic lighting circuits.

I've been confused by a two way lighting circuit and I'm after some help.

I have three down stairs lights, kitchen, lobby and toilet. They are all two way switched with two triple plate switch. They are wired in loop switch.

I would normally expect to see 3 x three core cables with wires between commons , L2's and L2's with the switch wires going to L1 and L2 but this is not the case.

Can an any one explain the style of wiring that has the Commons connected to L1s?

I'm assuming their is a convention or wiring style that I'm not familiar with.

Any house sparks offer any explanation or diagrams.