We'd love for each platform to receive an update as soon as we've finished programming it--we think we're making positive changes and want everyone to benefit ASAP! However, every time we are ready to ship an update, we can do so immediately on Android but must wait an unknown length of time (typically 1-2 weeks, but it could be more or less) for Apple's certification process.
At the time of this writing, we are currently making updates so frequently (9 in the first month!) that we can't wait 1-2 weeks each time for Apple to certify the update. So instead we're iterating as quickly as we can on Android, and trying to choose the most sensible versions to put through the Apple certification process. E.g., version 1.6 is an important update because it adds Challenge Events and 18 new bears. On the day it was ready to ship, we both released it to Android players and submitted it to Apple for certification.
(Note that iOS players do eventually get same changes that Android players get, but iOS players get in a single update the same changes that we rolled out to Android players over several updates. E.g., when an iOS player updates from v1.3 straight to v1.6, he will get all the changes we made in-between, too, and have the same features as Android players also on v1.6, even though Android had three updates in-between v1.3 and v1.6.)
Eventually the pace of updates will slow down, and at that point we may try to hold the Android versions for a few weeks so that versions for both platforms can be released simultaneously.