I have driven in China, US and central America using my iPhone (Google maps) and it worked fine everywhere even in place where there was no data or even no cell signal at all (such as in National Park in the southwest).
I suspect that they simply took their existing regional maps and made them freely available, without making any changes to them. Hopefully in the future they'll break the larger ones down into smaller chunks. For example, 99% of my activities are in the western US, so there's no value in me also having Canada, Mexico, and the Eastern US on my watch most of the time.
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