Anybody who bought stocks in April 1992 would still be cursing his luck while those who bought in April 1993 would be wallowing in clover. Similarly, contrast January 1999 (good) with January 2000 (bad).
This has nothing to do with stock-picking skills and plenty to do with commonsense asset allocation. If you pick a basket of sustainable, competently and honestly-run businesses, there will be little in the way of fundamental change inside a period of 12 months.
But there could be a huge difference in valuations depending on where prices happen to be. The adage about