The Google LINK: operator typically shows only a small sample of inbound links that Google knows about and/or counts. If you have a Google Webmaster Tools (WMT) account and have verified your site then WMT will also show you a list of inbound links that Google knows about and/or counts. Again, it is ONLY a sample. The WMT inbound link sample is usually bigger than the LINK: operator sample, but it's still a sample just the same. It is also worth noting that Google will sometimes include inbound links which are NOFOLLOW and/or inbound links from URLs known to be under penalty. In both cases, these links do not help in your rankings, but they still show up in the sample. Don't assume that just because they show up that they are being counted by the algorithm as a valid backlink. I believe that the reason Google does this is that they do NOT want you to be able to reverse engineer how their algorithm considers backlinks... They don't want you to able to determine which site's links are being counted, which site's pages are under penalty, etc. through experimenting w/ placing links and monitoring them with the LINK: operator/WMT. If Google showed you ONLY the links that were counted as well as ALL of the links that are counted then it would be simple to figure exactly how to manipulate the algorithm with backlinks. And since backlinks are probably the MOST influential ranking factor considered by their algorithm, the abuse of this particular ranking factor would have a HUGE impact on their SERPs. So they protect this information. Yahoo! Site Explorer will show an exhaustive list of backlinks using the LINK: operator... but those are simply all of the backlinks that Yahoo! knows about and NOT necessarily the same backlinks that Google knows about since they each have different URLs in their index. But it's a good approximation of your backlink profile as it might also be seen by Google.
Google shows only few samples of the backlinks buut now all. You can find all backinks at Google webmaster tools. Yahoo shows most of the backlinks.