The BDM districts in 1890 covered far more than the suburb
and the local deputy registrars maintained their local registers and sent quarterly returns to the Registrar General's Office, in Sydney CBD. That General Office included far more than just the Register for BDMs - The Lands Titles Office was perhaps its most significant "Registry" function. And, like most clerical admin functions in that era, things were done in strict alphabetical order. So BDM quarterly returns were sent through to the Sydney HQ. And there, these were sorted in Sydney Metro districts in Alphabetical order ... so for example, Ashfield would be a far way ahead of Burwood, and Burwood would be ahead of Canterbury which would be way ahead of Hornsby and it would be ahead of Petersham .... etc...
So each year would be reset to 'O' for the returns for those lodged directly at the Reg General's office (so mainly those born within Sydney District - a 'dense' but small district based around Sydney CBD) .... then the Sydney Metro Districts within the County of Cumberland - in alphabetical order -(so Penrith was probably the one immediately ahead of Petersham), and then the Rural districts ... eg Albury, Dubbo, Newcastle, Wagga, Yass.
HOWEVER, sometimes ... err ... the clerks at the district levels 'forgot' to submit their returns ... so a batch would be forwarded with the 'next lot' .... and until WWI there was no real admin protocol to follow up missing returns (and errr ... even post WWII births may have missed being submitted
promptly - I am aware of at least one rural district registry as part of the function of the Clerk of Petty Sessions - the entire register - decades - not discovered until EDP computerisation 1970s recalled all registers ....)
So 'high' numbers can indicate admin oversights
(across the state) but the online index is meant to include a feature to allow you to drill down to the actual date of birth rather than the date of registration.
Sorry for the long post, I hope it helps
JM