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You should think less in terms of "The rules say I can enter Singapore for 90 days" and more in terms of "The rules say the immigration officer can allow me into Singapore for up to 90 days." 90 days is allowed because it should be enough for the vast majority of tourist or business visits.
If you want to stay longer than 90 days (and staying for 80 days, leaving for less than a week and then coming back is, really, staying for longer than 90 days), then your visit doesn't match the profile of what visa-free access is intended for. Some countries are happy for people to make repeated long visits, as long as they leave briefly within the time limit of visa-free access. However, wealthy countries tend not to allow this: for example, the Schengen Zone only allows visa-free visitors to be present for up to 90 days out of any period of 180, the US doesn't allow you to make short visits to neighbouring countries to obtain additional visa-free time in the US, and the UK explicitly says that you're not to use short-term visas or visa-free access to effectively live in the country by making repeated long visits.
I'm not sure what effect your refusal will have on future visits to Singapore. It may be that you are no longer eligible for visa-free access; even if you can visit visa-free, it might be a good idea to get a visa, to avoid the risk of being turned away at the border again. You're unlikely to get a visa or be allowed to enter visa-free in the immediate future, so you need to make plans to spend your time somewhere other than Singapore.