Really I guess it depends on what the P50 is used for. I...

Really I guess it depends on what the P50 is used for. I wouldn't so much call it a sham, but more a possible outcome. For a company ranking several dozen prospects, an important aspect is going after the most valuable ones. The numbers are often wrong, there is a lot of uncertainty, but there really isn't a better way to calculate what potential volumes might be.

I wouldn't call it promoting company interest, small companies have to report these numbers, but it's IMO up to individual investors to be aware that these numbers aren't perfect. Calibration / normalisation is easier said than done. Geologists come up with conceptual models for what is going on and using that to make predictions (i.e. how are sands deposited, how much porosity, how has the oil or gas migrated there etc. The issue is that if you ask two different geologists to evaluate the same prospect with the same information, they'll come up with different estimates and different models to underpin these.
Possibly one of the main causes of optimistic bias however is not so much around being systematically wrong, but the company drilling a prospect is normally the one the most positive on this prospect (i.e. through a farmin process, or new acreage evaluation, those who have a less optimistic view often pass up the opportunity or get outbid. One possibility is that we are on average right, but we are sampling the optimistic views preferentially with the drill-bit. I've had several times when my P50 has been significantly below the P90 calculated by companies farming out.

It is possible for things to exceed pre-drill estimates, but once you have a discovery like SNE in a basin, it often causes people to re-calibrate geological inputs and expect outcomes like that local calibration point to be more of a norm. I have limited views on Samo without seeing the data, I see investing in exploration companies as gambling, and I've been to enough datarooms in my career to know there's always more complexity to any prospect than what is available to the average investor. The upside on SNE was in a bigger / prospect, judging by the pre vs post drill maps, better reservoir or more reservoir was expected, but a shorter column. Samo it looks like they are already banking on improved reservoir quality or thickness + the calibrated understanding of column heights based purely on fitting more recoverable oil into a significantly smaller structure in the P50 case, without knowing what geological model or geophysical data this is based on I really can't comment on my own expectations on Samo. I'd note that SNE North and FAN south apparently significantly below FAR's estimates so FAR has also gotten low side outcomes.

M&A and exploration are usually dealt with very differently from tax points, as a lot of countries have preferential treatment for exploration activity. For most US companies for example, every dollar spent on exploration anywhere globally, they can often get something around 60c back assuming they have income to offset it against. Australia also has rules about Australian exploration against Australian taxes (doesn't transfer to overseas exploration). M&A doesn't get this treatment, so even if a company did decide to pay for exploration upside, that $50 million dollar carry would likely get valued at around $20 million in cash.