Question about Synthesis and fitter

Synthesis translates the RTL to FPGA resources, and is device specific. So, for example, the Cyclone IV has 4-input LUTs, so it will map to that, while Cyclone V has ALMs(Adaptive Logic Modules) and it will map to those. The fitter takes those resources and places them in the FPGA, optimizing for the best timing and routability(and other things, but those are the main two). Synthesis can give a pretty accurate estimate, but the fitter will give ideal. For Cyclone IV, it's straightforward and easy to understand. The Adaptive Logic Module is more complicated because it is adaptive, and therefore synthesis can target different size LUTs and the fitter can pack them together differently, so there's some variation, but it shouldn't be too large.