cron_last vs node_cron_last variable
The only place node_cron_last is set is in _node_index_node(), which is run when indexing a node for search results.
Its purpose, per the comments:

Save the changed time of the most recent indexed node, for the search results half-life calculation.

cron_last is simply used to

Record cron time.

per drupal_cron_run()
Given that REQUEST_TIME is used for all values (cron variables + node changed date) in the same request, you might well expect those two variables to contain the same value.