They're reverting genetically to parental varieties. But: was the change in the tomato shown by a single specimen, or are you talking about seedlings coming up different from the seed source plants? Because in that instance the seedlings could even have crossed with other tomato varieties in the vicinity. In addition to the fact that these are separate genetic individuals from the seed source plants - seed producers use re-selection techniques to keep seed strains true to type.

• The white color hippeastrum is the same bulb, the flower shape didn't change, except the color. Now I have pretty nice dark Royal Velvet. The tomatoes: I had two bushes from the previous year seeds. Both were red. The taste was the same as when they were white. Several years ago I had taken a Dutch Fair tulip bulb to Europe and a year later it bloomed true to the name (yellow-red stripes). It looks like a different continent also plays a role. Thank you for your answer.