Perhaps the Mishna in Avos is consistent with all three opinions in the hagadah. All three opinions in the hagadah must agree that there were ten plagues in Mitzraim, yet they argue if there were ten, forty or fifty. The idea is obviously that each plague can be counted for more than one, if broken down into more detail. This would mean, that the given is that there were sets of ten and ten plagues, then the question would be how many each plague of each set would count for. And I'll attempt to prove this.
If you notice, the one to five ratio is derived from the fact that the plagues in Mitzraim are referred to as the finger of G-d and the splitting of the sea as His Hand. This may be hard to understand, since only one plague was referred to as a finger of G-d, not all of them combined, so perhaps the Mitzraim Plagues totaled ten, and the Sea Plagues totaled five?
The answer is, that the actual amount of plagues, when counted broadly as in the Torah, is non-negotiably ten and ten. The question remains as to the severity of each set. So if a Mitzraim Plague can be called a finger, and a Sea plague a hand, then the consensus is that these refer to severity and not amounts, and it can be inferred that the ten of the Sea can be counted as five times that of Mitzraim.
[sorry, this is just an oversimplified version of Alex's answer, with a proof]