All three panels hold the same numbers. Panel (a) gives each age group its own bar, which is the arrangement to use when the question is how one group compares with another. Panel (b) stacks the groups into a single bar for each year, so the height is the whole population and the question becomes how the total is built up. Panel (c) stacks them again but scales every bar to the same height, so only the shares remain.
Reading across the three panels shows what each choice hides. Panel (a) makes the growth of the working-age group obvious but says nothing about the total; panel (c) makes the rising share of the over-65s obvious but conceals that every group is growing in absolute numbers. The slider picks out one year across all three panels at once.