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If a part is offered in both industrial and commercial grades, it is possible that the commercial parts are dropouts from testing to industrial grade.
However, it depends on relative volume of both grades, as dictated by customer demands.
Making industrial-grade ICs requires tighter design to accommodate wider process corners and use of more expensive foundry libraries, and fabbing also cost more. It also requires more conservative layout technique, wider interconnect, double vias, etc. To achieve good integration, a foundry usually offers a set of standard cell libraries for every taste, but the cost differs as well.
If majority of customers for a particular IC demand industrial grade, the IC will be designed and fabricated as such, and test dropouts would sold as commercial ICs.
Getting to industrial grade might also include a change in IC packaging, to use, for example, a metal-ceramic instead of plastic.
However, if the volume for commercial grades is high, the die can be taped out with less expensive libraries and manufactured by less expensive variant of the fab process, so save on cost and maximize gross margins. Then the same functional IC will come from different wafers for different grades, and will go through a different set of testings.
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