Why does the F16 HUD give you one number for your speed and the analogue gauge another?
The primary airspeed indication is in the HUD. You can select CAS (calibrated airspeed), TAS (true airspeed) and GND SPD (ground speed) on the HUD panel on the right console. 

The backup airspeed indicator (the analogue gauge you are referring to) is located below the ICP (integrated control panel) on the center console. It can indicate from 80 kt to 800kt with a scale in 10 kt increments up to 400 kt and then 50 kt increments up to 800 kt. It is a rather basic analogue instrument providing IAS (indicated airspeed) only. 

As the scale is very coarse (higher precision is not needed) you won't be able to see a noticeable difference when the HUD is switched to CAS. Most probably the HUD was switched to TAS or GND SPD. But without further information or an example photo from your side it is hard to tell.