Reading the Gospel of Mark, the beginning chapters brought to mind an old, distorted view of Holy Scripture that 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. He tells us at the start (Mark 4:30) that this is a parable. 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).