I got sarnath'd while writing this post, but here's my in-depth explanation:
Relevant Rules wrote:
613.3a Layer 7a: Effects from characteristic-defining abilities that define power and/or toughness are applied. See rule 604.3.
613.3b Layer 7b: Effects that set power and/or toughness to a specific number or value are applied. Effects that refer to the base power and/or toughness of a creature apply in this layer.
613.6. Within a layer or sublayer, determining which order effects are applied in is usually done using a timestamp system. An effect with an earlier timestamp is applied before an effect with a later timestamp.
613.6a A continuous effect generated by a static ability has the same timestamp as the object the static ability is on, or the timestamp of the effect that created the ability, whichever is later.
613.6b A continuous effect generated by the resolution of a spell or ability receives a timestamp at the time it’s created.
Neither ability is a characteristic-defining ability, so they both apply in Layer 7b. The timestamp of
Myth Realized's effect is the timestamp of the effect that created that P/T-setting ability, which means the time the activated ability resolved. The timestamp of
Starfield of Nyx's effect is the timestamp of
Starfield of Nyx itself. This means the Starfield's effect should be applied first, and then the Myth's effect should be applied, so the resulting P/T should be equal to the number of lore counters on
Myth Realized.