Firstly, Toyota make the best hybrid systems and have been doing it longest. They're extensively proven.
The computer has to manage a series of factors to decide what is the best form or propulsion at any given moment. These often include factors that might not instantly spring to mind.
For example, one of the important elements is to get the catalytic converter in the normal exhaust system up to operating temperature as quickly as possible, so that when it is using the internal combustion engine, it is doing so with minimum emissions. This will require the engine to run at times when logic or owner perception suggests it shouldn't.
If after a period of using mostly electric power, the catalytic converter can cool down to a degree where it needs to run the engine just to keep it within the optimal operating temperature window, often at slightly elevated revs.
Also, as the system uses both a regular battery and the main hybrid batteries, these also need to be kept at optimum charge. The petrol engine not only contributes to the drive of the car, but also acts like a generator to recharge the batteries - again at times which might not seem logical.
Hybrids are at their most consistent when they are large and joined to a big engine like in a big SUV etc, where much of the variance can be smoothed out and blended more seamlessly.
On a small car such as a Yaris, there is less scope to 'blend and absorb' the levels of charge and power capacity, so you will notice it switching between power sources and 'doing its thing' more readily.
As the transmission that blends the electric motor and petrol engine together is very different to a conventional gearbox, it, by its very design will permit and encourage the use of higher engine revs than in a normal car at times, especially when accelerating, as it chooses the desired power level (as requested by your right foot) and sets the engine at the revs required to deliver that power and just holds it there until your power demands have been satisfied.
Overall, they are great, but do come with some quirks and idiosyncrasies that can take some getting used too when compared to a conventional car - all of which are highlighted and slightly exaggerated on a smaller, compact hybrid system.
Hope that helps.