The pyramids of numbers shows the raw number of each species at each trophic level. The top example is a typical food chain with a large number of producers but diminishing numbers of consumers. However, if the producer was a tree, followed by insects, then the bottom bar would appear small as many organisms feed on one tree. In this instance the pyramid of biomass is more useful as the tree is much larger.
In the lower example, both the pyramid of numbers and biomass show a smaller producer bar; given what was discussed under the previous heading - this does not make sense. This is because the phytoplankton reproduce very quickly. However, when we represent this information in a pyramid of energy we get a true pyramid.
Plotting the energy will always give a true pyramid because it is impossible to create new energy so a trophic level will always be smaller than the one below it and as discussed above there is a great deal of energy loss at each level.