docs: correct counted-SBC decay amortised cost figure

docs/bitbrain_counted_sbc.md

@@ -61,7 +61,7 @@ alternatives for three reasons:
   The bitset `infer` is unchanged.
 
 **Config (which is which).** The default is and remains `smBitset`; the bitset
-path is byte-for-byte unchanged. Counted mode is selected explicitly or at
+path's behaviour is exactly unchanged. Counted mode is selected explicitly or at
 runtime:
 
 | knob | kind | values |
@@ -92,8 +92,8 @@ The gun-sized byte-per-cell cost is 12 MiB — acceptable for a gun (and the
 whole 12.1 MiB model fits comfortably next to the rest of a bot). The MNIST
 240 MiB is fine for an offline measurement machine but is 8× the bit tensor;
 packed 4-bit (INFERRED, not implemented) would halve both figures. At
-MNIST scale, `decayEvery = 2000` costs ≈ 0.2 ms/learn amortised (measured train
-30.3 s vs 25.4 s over 60k for the decay arm, i.e. ~0.08 ms/sample extra).
+MNIST scale, `decayEvery = 2000` costs ≈ 0.07–0.08 ms/learn amortised (measured
+train 28–30 s vs 24–25 s over 60k for the decay arm).
 
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