I don't have the specific breakdown of the offer Bell refused. I can't find it anywhere; so I've been using the numbers put out there that haven't been refuted: 5 years, $70M, $10M guaranteed, $33M in rolling guarantees over the first 2 years, $45M in rolling guarantees over the first 3.
His salary for the first 2 years couldn't total more than $23M (if the rolling guarantees of the first 2 years=$33M & the signing bonus was $10M). In that case, it really depends on how the salary structure is divided for the the first 2 years. If year one's salary wasn't much more than $4.5M, Bell doesn't gain much. He was already guaranteed $14.5M when he signs the franchise tender (and he'd be obligated to report to TC on time). Using Spotrac, most of the Steelers long-term contracts (Brown, Roethlisberger, Pouncey, Heyward, Villanueva, etc) have really low first-year salaries ($1M-$3M range). Assuming this to be the case in the Bell contract offer, it stands to reason that the Steelers total 1st-year pay out to Bell wouldn't have been above the $14.5M number (and possibly below it). That means that they could have cut him prior to the 2019 season (whatever date the rolling guarantees kicked in) & only been on the hook for the 2nd half of the $10M signing bonus. Bell would have earned less (possibly the same) as he'll make this year on the franchise tag. Good deal for the Steelers, not so much for Bell.
Gurley's contract, however is different. If he is cut prior to the 2019 season, LA takes a cap hit of $16.8M, and Gurley makes almost $22M for 1 year (2018). That won't happen. His rolling guarantee for year two is virtually a lock. His 3rd year rolling guarantee is pretty safe, as well, but not nearly as much as year two.
I get why the Steelers offered what they offered, it made sense for them.
I get why Bell turned it down, it didn't make sense for him.