Why would estoppel allow officials to play fast and loose with legal rules and the limits of a public authority’s powers, duties or jurisdiction?

Question

I read all three cases in footnote 193, but they don't explain the boldening.

      At an earlier time, the courts rejected the use of the private law doctrine of estoppel in easing the plight of someone who had relied on an assurance from an official only to find that it was not binding because it was thought to allow officials to play fast and loose with legal rules and the limits of a public authority’s powers, duties or jurisdiction.¹⁹³ Thus estoppel could not affect the obligation to perform a statutory duty. The House of Lords reconsidered the issue in 2002, in which Lord Hoffmann said:

It seems to me that in this area, public law has already absorbed what is useful from the moral values which underlie the private law concept of estoppel and the time has come for [public law] to stand upon its own two feet.194

As a result, the analogy between legitimate expectations and estoppel is not one which is likely to be of any utility in the near future.

¹⁹³Maritime Electric Co v General Dairies Ltd [1937] AC 610; Rhyl UDC v Rhyl Amusements Ltd [1959] 1 All ER 257; Essex Congregational Union v Essex CC [1963] AC 808.

Bradley, Ewing. Constitutional and Administrative Law (2018 17 ed). p 667.

Answer 1

Presumably the fear was that a law or policy governing the public authority might say one thing, but if an official assured a member of the public to the contrary, then the MoP could then require the authority to comply with the official assurances instead of their policy. Thus the official could unilaterally force the public authority to change its behaviour in arbitrary ways.