I believe if one could look at the die of the processor itself, the location of the power and Gnd connections would show power & gnd coming in to various sections of the chip, perhaps to help with distribution. I know AVCC goes to the corner of the chip where the ADC circuit is for example.
In SMD parts, there are multiple VCC pads. In DIP parts, the die pads may be wired to the same leg. Or perhaps only one, as Atmel says the current per VCC pin is 200mA - DIP 1284 only has 1 VCC pin, while SMD has 4.