I don't know if they produce their numbers at other times (not really interested in that aspect of their data). Usually if the manufacturer produces numbers, third parties will only release their inferred numbers in cases where the manufacturers numbers may not be reliable. For example where a manufacturer counts shipments to retailers as sales. Otherwise its largely a pointless exercise when there's a more reliable data set available.
In the absence of manufacturers own data such inferred data, where there is likely to be a strong correlation, is useful for shareholders/investors/market analysts/etc. Say Apple and Samsung were the other way around this year, you may well have seen a dump of Apple stock. It may also have pushed them to release their own numbers.
For Flurry themselves its a marketing exercise of course

. Whilst you may be sceptical, Flurry's core business is collection and dissemination of data. Moreover, they are also heavily involved in ad tracking/monetisation, so advertisers rely on them to track impressions/clicks. Therefore, I doubt they would engage in any nefarious activity for the sake of a bit of publicity. If caught it would call into question their credibility which would impact their core business. If they've released these figures, they must believe they are representative.
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