An old, American, distorted view of Holy Scripture can cause confusion for some. This is another failure to understand context. This troubles some modern, technical minds. We all find it hard to make the paradigm shift to appreciate the Semitic mind.
The problem: “a mustard seed which…is smaller than all seeds on earth…”
This tiny seed is less than half the size of a poppy seed. Still, a botanist or flower gardener can show us smaller seeds, but that is entirely irrelevant. Jesus is not speaking as a botanist. Mark tells us at the start (Mark 4:2) that these are parables, here. In parables we find hyperbole, e.g. the camel (or rope?) that cannot pass through the eye of a sewing needle [forget the ‘urban legend’ of the gate] but may be swallowed instead of a gnat!
Understanding context keeps us from focusing on gnats.
For the technical mind, “all” must mean “all.” But for the literary mind of the writer, it is a device to convey the point, as in the opening of Mark: “Then all the land of Judea, and those from Jerusalem, went out to him [John] and were all baptized…”
No one thinks that if we had the exact census numbers for Judea and Jerusalem of that day, that they would equal the number of all those who went out to hear John or the number of baptisms.
Back to the mustard seed, this extended simile, a parable, makes a vivid point. And the Jewish proverb (Plummer), “Small as a mustard-seed,” is used by Jesus in comparison with the resulting bush to emphasize “the sheer miracle of the growth of the Kingdom” (Albright).
30 And he said, “With what can we compare the kingdom of God, or what parable shall we use for it? 31 It is like a grain of mustard seed, which, when sown on the ground, is the smallest of all the seeds on earth, 32 yet when it is sown it grows up and becomes larger than all the garden plants and puts out large branches, so that the birds of the air can make nests in its shade.”
33 With many such parables he spoke the word to them, as they were able to hear it. 34 He did not speak to them without a parable, but privately to his own disciples he explained everything.–Mark 4 ESV