Just reread https://steemit.com/faq.html#What_do_the_dollar_amounts_for_pending_payouts_represent

Is the disparity because  the estimated value  steemit uses assumes SBD to US$ parity? Ie it's basing it's estimate on each SBD being worth US$1 rather than the actual current value of about US$3.57? 

I noticed recently that in fact SBD was selling for much more than STEEM itself.

Whilst at this, based on the following FAQ answer there appears to be some way of automtically converting SBD and STEEM between each other. Yet I've only ever been able to do it on the market.  Is it possible to do this conversion without going to the market?
https://steemit.com/faq.html#How_do_Steem_Dollar_to_STEEM_conversions_work

Best

@yabapmatt ok I think I might finally understand.  Based on this helpful post it seems that steemit lists the reward value of a post in SBD and SBD's are worth more than steem hence the disparity.

https://steemit.com/steem/@yabapmatt/how-to-calculate-the-value-of-a-vote