Volvo's I-Shift is not a hybrid system, it's a gearbox similar to an American 18-speed manual truck gearbox. The gear stick and clutch are replaced by pneumatic actuators and the operator's brain with a computer and sensors. It's available in Volvo trucks with a large number of options including additional crawler gears, dual-clutches and hybrid drive.
Volvo hybrid buses use the same hybrid system as Volvo trucks, but with a basic I-Shift gearbox with no options. Between the clutch and gearbox is an Integrated Starter Alternator Motor (I-SAM) which can be used to start the engine (only while in neutral), charge the batteries and drive the wheels. When starting off, the computer picks a suitable gear and sets off with just the electric motor. At the end of that gear (or if more power is required), the engine is started and further driving is performed with both the diesel engine and electric motor. Braking is supplemented with the electric motor to recharge the batteries. When the handbrake is applied the engine will usually stop, but if there's not enough battery charge the engine will keep running, driving the motor while in neutral to charge the batteries.
Your system sounds quite similar, except it doesn't use a transmission with noticable gaps. The engine revving up when braking is not to help charge the batteries but to provide engine braking, which is not uncommon on conventional diesel vehicles to reduce brake wear. I would guess that the electric motor is after the gearbox and has a maximum speed of about 450 rpm, so stops being effective as either a motor or brake around 40 mph. It's interesting you describe it as a CVT as they're not very common in buses, at least over here - perhaps you could provide more info about the make/model so I could look it up?
Non-Volvo buses tend to use the same system as yours - essentially a conventional diesel bus with an electric motor for low-speed driving. I would imagine they take the place of a retarder (turbine that spins in oil to slow the vehicle without using the brakes), hence the need for engine braking at high speeds.
These hybrid systems are "parallel" hybrids - both the engine and electric motor drive the wheels mechanically. The New Routemaster (NRM) in London uses Siemens' ELFA2 hybrid system, which is a "series" hybrid - the engine is just a generator, with the electric motor doing all the driving. These buses have no gearbox at all - the engine runs when required, independently of road speed. This system is used in a number of buses on the continent too, but I don't think series hybrids are as common as parallel hybrid vehicles.
As for why different countries use different systems, it's up to the operators which buses they buy, and manufacturers which system they use. Volvo already had a truck hybrid system, so it made sense to use it in their buses. It has gaps between gears (especially on earlier vehicles that used the I-SAM as a starter motor), but the "manual" transmission is better for fuel economy. Other manufacturers just modified their existing conventional diesel buses because it was easier, cheaper and faster to develop. Some realised that they'd need to make electric buses in the future anyway, so they got a head-start by making electric buses with generators until battery technology caught up - some early electric buses had enough capacity to drive the bus but not enough to also run the heating, so they had a diesel heater.