The BNC/COCA lists derive from an harmonization of the original BNC lists with a new set of US lists based on Mark Davies' (Brigham Young Univ.) 450 million word Corpus of Comptemporary American English (2012).
For all the brilliance of the pedagogical version of the BNC lists developed by Paul Nation in 2005, its exclusively British composition presented words like 'bloke,' 'chap,' and 'lorry' as 1k or 2k items, and this left North American teachers and learners perplexed. Still, these lists were an excellent placeholder until an international frequency list could be developed - as has now happened.
Nation and Davies have integrated BNC and COCA lits into a single master list and at the same time expanded the offering to 25 k-levels. The new list is built according to the usual criteria of both frequency and range across sub-corpora, now joined by co-representation in both North American and UK varieties of English.
These new frequency lists have been used on Lextutor tel quel (as is, un modified), and BNC will still be available as an option. Changes will mainly affect VP, Frequency Trainer, Familizer, and 1-Word-VP.
The vast majority of lexical items remain at the same frequency level or perhaps shift by one level - users should explore.
The goal of refining these lists include as usual to drain the swamp of 'off-list' items, to get more text coverage for fewer items, to get a better fix on what Norbert Schmitt has called 'mid-frequency vocabulary,' to get a baseline for potential 'technical' or 'domain specific' items, and to do more on Lextutor for the advanced learner (a growing constituency).
For an update on the BNC-COCA project, look here.