Determining what is a "match" is not necessarily that easy as there can be false positives and false negatives. All the companies set a minimum threshold for declaring a match so in theory there will be some false negatives that don't meet the companies' thresholds. However, there is a bigger problem with false positives. This is especially the case when the "match" is only on a small segment, and particularly on segments that are less than 15 cM.

There are now additional problems because of the different SNP microarrays used by the testing companies which have a low overlap. Companies have special techniques such as phasing and imputation to try and overcome the low overlap but there is inevitably a high false positive rate on the smaller matches.

Here are a few articles for further reading:

https://thegeneticgenealogist.com/2017/12/29/a-small-segment-round-up/

https://isogg.org/wiki/Identical_by_descent

https://isogg.org/wiki/Autosomal_SNP_comparison_chart

In general the matches at AncestryDNA are the most reliable as they have the most sophisticated algorithms backed up by solid science.