AMD CPU's allocate memory differently. To be vulnerable a hacker would have to predict or search each of the memory allocations in an AMD machine. There are millions. This is like searching for a needle in a haystack.
On an Intel machine, the answer is helpfully provided on a silver platter, you don't have to go looking.
That's the difference.
When AMD says they're vulnerable, they mean that yeah technically if you searched through the entire memory bank you may be able to find the exploit. TECHNICALLY, even though it's really difficult and nobody would do it because well, Intel is an easier target and there are more Intel's around. It's like running from a Bear.. no need to run faster than the slowest person. With respect to Spectre, Intel is the slowest person.
“It is much more difficult on all AMD CPUs, because BTB entries are not aliased - the attacker must know (and be able to execute arbitrary code at) the exact address of the targeted branch instruction.” https://www.reddit.com/r/Amd/comments/7o2i91/technical_analysis_of_spectre_meltdown/