Take the 1983 general election. Labour and the Alliance (LibDems) got almost the same share of the vote nationwide - 27% and 25% respectively.
Now, as a rule of thumb, you need about 40% of the vote in a constituency to win it and get the MP. Strictly you need 50.1% of course, but if the votes for the other parties are split you just need one more vote than the next most popular.
However, the Alliance vote was evenly distributed - they got about 25% everywhere, which meant a lot of second places (eg Lab 40%, All 25%, Con 25%, Others 10%) while Labour got a lot of 40% type votes in some areas and 10% type votes in others - so a lot of first places and a lot of third places, which average out to 27% but is much more concentrated in particular parts of the country.
Result: 203 Labour MPs, 23 Alliance, for almost the same number of votes.
Similarly, Plaid get hardly any votes at all, but almost all their voters live in four North & West Wales constituencies and so they get 3-4 MPs in most elections.
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Now, as a rule of thumb, you need about 40% of the vote in a constituency to win it and get the MP. Strictly you need 50.1% of course, but if the votes for the other parties are split you just need one more vote than the next most popular.
However, the Alliance vote was evenly distributed - they got about 25% everywhere, which meant a lot of second places (eg Lab 40%, All 25%, Con 25%, Others 10%) while Labour got a lot of 40% type votes in some areas and 10% type votes in others - so a lot of first places and a lot of third places, which average out to 27% but is much more concentrated in particular parts of the country.
Result: 203 Labour MPs, 23 Alliance, for almost the same number of votes.
Similarly, Plaid get hardly any votes at all, but almost all their voters live in four North & West Wales constituencies and so they get 3-4 MPs in most elections.