Sounds like they have a longer then usual striking voltage time.

The striking voltage is much higher than the operating voltage in any fluorescent bulb, and is normally only generated for long enough to get the discharge in the bulb started.

They may be holding the voltage higher until the bulb has warmed slightly.
A common complaint of florescent bulbs is they can take a few minutes to come to full brightness once turned on.
The gas inside the tube needs to warm up.

I have some Sylvania micro sized 23w (100w replacement) bulbs that are doing this. Don't know why.

"I have some Sylvania micro sized 23w (100w replacement) bulbs that are doing this."

Do they then slowly brighten back up over a few minutes?

Yes, but they start out *very* dim. The Philips micro-sized 100w-replacement CFL bulbs start but much brighter than the Sylvanias do, but also give out a slightly pinkish cast whereas the Sylvanias look more like incandescent bulbs when they finally warm up (although I notice as I write this that my last batch of Philips CFLs claims higher brightness than my ones from last year, so maybe they've changed; I'll report back here if they have).