But if the 85kwh batteries were actually 100kwh ones, then why not advertise it at the launch like the 70D?
I don't believe there are secret 100s out there, but here's why I think the 70D may have been rolled out by itself, with an 85 kWh replacement delayed a few months:
The 60 kWh version of the car was a small fraction of Model S orders (IIRC Tesla has said 10% or less, Consumer Reports said 2%). Thus, the introduction of the 70D as a 60 replacement was going to ruffle the feathers of a rather small group (compared to 85s) of new owners and those with cars in the process of being built and shipped. With planning Tesla could have minimized that further by timing the 70D introduction to coincide with a time 85, 85D and P85D comprised all of recent and current production (i.e. a time they'd not produced 60s for a couple of months, so very few would have just gotten one delivered).
Similarly, it's possible that Tesla is building it's order book of 70Ds now so that
if/when a larger battery replaces the various 85s, they can be in a period of predominantly 70D production and cars in transit. For example, if a larger battery is introduced on June 30th (available for order, or change in reservation that day), and Tesla has 6 weeks (~6K) worth of 70D orders to fill, they could have shut down production of 85s 6 weeks earlier in Mid-May in favor of strictly producing 70Ds, so essentially no 85s are in production when a bigger battery is introduced, and as small a number as possible are in transit. This would minimize abandoned/refused 85s, and somewhat minimize the amount of people who would have gotten their new 85s within a few weeks of such a potential new battery introduction. Moreover, those who did recently take delivery of any type of 85 would not be so shocked (and hopefully somewhat less irritated) to hear of a 100 or 105 kWh version coming out given that the 70D rolled out 3 months earlier.
As others have written here, I doubt Tesla will announce a larger pack than the 85s for the S, until they are ready to start making them. I doubt that this will be so in the next couple of weeks, but I'm cautiously optimistic for something this summer (I agree with others that the Model X is very likely to have a bigger pack, and we expect to hear X details by some point in the summer).