In terms of ecological pyramids, how can pyramids of numbers sometimes be a different shape from pyramids of biomass, even for the same food chain?

Ecological pyramids are a way of visualizing a food chain, either with the mass of organisms or the total number of the population of organims at each stage of the food chain. Pyramids of biomass are always a "classic" pyramid shape whereas pyramids of numbers can sometimes be inverse or have levels of varying width. This is because mass and number are not equal to eachother because oragnisms can vary greatly in size. Think of an oak tree. One oak tree has a very large mass and therefore a pyramid of biomass would be wide and support higher stages, such insect primary consumers. However, a pyramid of numbers would have a very small base as it is only one tree, but would still supports hundreds of insects as these have smaller biomasses and so would be inverse in shape.