If you're going to set your stall out by what the polls are telling you then you're going to be very confused by upcoming events in the next few years.
Only if you read the headlines.
Your quoting snapshots in a moving process, people change their opinions over time, it's like asking you what you want to eat in the evening then serving it to you for breakfast on Sunday.
On Brexit it was always going to be close, a close vote encourages the losing side to get out in greater numbers and adds complacency among the leading side. The Brexit vote also included a significant protest vote which is hard to measure.
Trump as said, was a 25% chance of winning on election night, lost the popular vote and made it in by about 300k votes - about spot on
May was in a very strong position until she called the election and then screwed up her campaign, the polls moved all the way through the campaign. The amount of undecided swayed that in many areas.
That was all in the poll data and analysis, just not if you read the first number.