That is a common misconception. There are three kinds of common solder:
1) silver solder-used in high strength applications, polarizes over time making it resistant to current flow
2) plumbers solder-need I say more, normally has a higher melting temp that can result in damaged electrical components and isn't as good of conductor as electrical solder
3) electrical solder-usually a 60/40 tin/lead mix with a resin or flux core, formulated for lower melting points and better conductivity. Electrical solder also cleans the surfaces being soldered due to the resin/flux core for better adhesion and lower resistance
Something else that can come into play here is whether the surfaces being soldered were "tinned" or not or if the joint is "cold". Tinning is nothing more than applying a thin coat of solder to what is being soldered together or, in the case of stranded wires, filling in between the strands to make the wire more like a solid wire. A cold joint will increase the resistance due to less surface contact from improper application. When done, solder should be shiny and smooth, not rough and grey