If your scale measures to 1 gram, then you cannot know (using the scale) that the yeast weight 2.5 grams. Am I missing something.
Trust the Red Start packet weight. It is very standard and standardized. I assume the 3 pack looks like the picture below and each packet says 7 grams / 0.25 ounces. Then, yes, each packet weight is 7 grams. Anything is possible, like a big time manufacturing/packing glitch - I would guess, however, you are not measuring correctly.
You can also measure with measuring spoons. 1 teaspoon is right about 3 grams. So, 1.3 grams/3 grams is .43 teaspoons - right in between 3/8 teaspoon and 1/2. Should be close enough.