No artist consistently produces work of the highest quality. Turner, Picasso etc etc all had their off days, but in the past I suspect because art had less monetary value, poor works were discarded. Now celebrity culture means anything a famous artist has drawn has monetary value.
I recently saw the newly discovered Van Dyck portrait at the Bowes Museum in Barnard Castle. To be perfectly honest I thought the quality was poor, not a great portrait, which is probable why it had drifted into insignificance in the years since it was painted.
The same applies to poetry, which I read, if you get an anthology of one poet's verses and try to read all it will soon be found that all the great poets, unless they were very self disciplined and regularly self edited placed their pearls among an awful lot turgid rhymes.