The assumption itself is not correct.
Gross salary or CTC is a function of Basic pay. (mathematically speaking).
Any change in Basic pay is reflected on Gross Pay or CTC.
Here, Gross Pay and CTC are dependent variables of Basic Pay.
However, small companies, without any well-defined salary structure and compensation system; simply try to offer new candidates Gross Salary higher than what they are getting in their own company (which is logical); and then they work backward and try to retrofit a Basic pay, calculated from the Gross Pay or CTC.
If that is what needs to be done, then it hardly matters, whether one is arriving at the basic Pay from the Gross salary or CTC. In both the cases, basic pay is just an arbitrary figure.
In good and big established companies, Gross Pay and CTC automatically gets calculated from the Basic pay (and the Grade/Job Band) of any employee.
Instead of just fixing Basic Pay randomly, HR's in companies not having any Compensation system, should propose Grade/Job bands and corresponding sets of Basic Pay with annual/performance increments along with a well defined system of Other Allowances, Reimbursements and Perks.
I hope members will prefer to have a well thought-out and well-defined System in place, rather than having an arbitrary system.
Warm regards.