There could be a difference in which duration is being reported, T.1 vs T.5; T.1 is typically ~ 2-3x longer... typically T.5 time is reported. It's also likely that the differences are due to bulb/electronic design differences.

But I think there may be some issues with the premise of your question.

Thyristor controlled flashes have their fastest duration at full power; but it is typically still rather slow. The fastest durations come from LED based flashes, but at very low power. And with modern IGBT flashes higher duration speeds also occur at lower output power; none of those really help you to overpower the sun by using faster SS's.

Most modern IGBT speedlights have a T.1 time around 1/250 and a T.5 time of ~ 1/1000. For instance the SB700 has a T.5 of 1/1042 and a T.1 of 1/305, so I think your information is suspect.

https://gock.net/blog/2012/flash-durations-small-strobes/ https://improvephotography.com/46974/flash-duration-speedlights-long-pop-flash/ https://www.konsul-instruments.com/en/flash-duration-gallery/

And the flash duration doesn't have much to do with the shutter speed you can use; that is controlled by shutter travel time. And most focal plane shutters (mechanical, electronic/rolling) are relatively slow at around 1/250 or longer.

There are two exceptions to where flash duration does relate to usable shutter speed. As you mentioned, aperture (Leaf) type shutters is one; but that is not found in modern DSLRs to my knowledge... it is found in some small sensor/compact cameras. The other is using tail sync which is a dumb (no communication) unsync'd technique which has other drawbacks; such as needing a manual timing offset to correct the sync, and fading across the image. A third potential exception could be a global electronic shutter; if they ever implement those.

But those exceptions are rare at best, or something of a work-around trick; they are not commonly available/used to overpower the sun so there isn't a lot of reason to design for it.

A common method to overpower the sun is to use ND filters which allows you to use slower SS's and full flash power. And with modern IGBT flashes high speed sync is common, which causes the flash to act as a constant light source (at reduced power) allowing the use of higher SS's.