To explain that in graphical manner, here is the Pouring Test result in the image. The oils are poured in different temperature to see how well they flow. Almost the test concept with oil drop test on sloping surface to see how well the 'sperm' swim. The less viscosity at certain temperature pour better compared to thick/ higher viscosity oil at their respective temperature.
The 'problems' with monogrades are they tend to thining too early when engine heated if using low viscosity monograde i. SAE 10. In contrast, the high viscosity monograde i.e. SAE 30 might be able to withstand thinning or losing lubrication is too thick during engine at cold temperature.
The marvel of multigrade i.e SAE 10w/30, 5w/40 is to come in as the mediocre in the not too thick during cold or winter and not too think during engine peak hot temp.