The use of such "pensionable salary" figures goes back to Defined Benefit pension schemes where the eventual pension was calculated to include the State Pension, e.g. "With 40 years' service you'll receive a pension of 50% of final salary, inclusive of the State Pension." That was far cheaper for the employer to fund for than funding for a full 50% of final salary.
It looks like your employer has continued this terminology to reduce the cost of their contributions to the scheme. There's no obligation on an employer to make any employer contribution at all, so if they choose to make a contribution of x% of pensionable salary, rather than x% of real salary, that's up to them.
That said, the fund manager's projection should reflect the true position.