The problem is - how do you define an mtDNA match?
If your mtDNA test was anything less than a Full Sequence test (or equivalent), then you only have a haplogroup for comparison. If your haplogroup matches someone else, you can say you have a haplogroup match, but the mtDNA haplogroups are VERY old, thousands to tens of thousands of years old. It's really hard to call a haplogroup match a true match if it cannot provide match information in genealogical times. An ethnicity determination is almost always more current than an mtDNA haplogroup.
To truly compare mtDNA results, you need to know both the most detailed haplogroup possible *and* your extra mutations. That is what the GD (Genealogical Distance) is based on. GD essentially represents the difference between 2 mtDNA tests. A GD of 0 means they are genealogically equivalent, and can be called a true match. A GD of 1 means there is one difference, and it is questionable whether you can claim that as a match or not. mtDNA mutations are extremely infrequent, averaging about a thousand years between them, within the general range of one generation apart to as much as 2000 years apart.
I think most would agree that matches are generally defined between people traced in genealogical times, and that's usually considered to be between the last 300 to 500 years. So to call a GD of 1 a match, I believe you should have evidence that pinpoints the common ancestor who first had that differentiating mutation. That probably eliminates most GD of 1 'matches'. It's unfortunate but most GD of 1's are likely to be related by someone who lived at least 1000 years ago, and possibly as much as 2000 years ago.
Your extra mutations really matter, because each one that's different can represent a thousand years of difference, from others with the same haplogroup.
This helps explain the difference in the numbers of 'matches'. It's not just the difference in population sizes, or the difference in quantities of testers within each haplogroup, important as those are. It also matters how far back the last mutation was. A mutation that happened 1500 years ago will have many many more descendants than a recent one.
