user16626: Here is a short game played online at chess.com.
[FEN ""]

1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. c3 d6 5. d4 Bb6 6. dxe5 

Black resigned for some unknown reason; maybe a mouse slip or he/she needs to leave. When the game was analysed at chess.com, the game accuracy of white is 99.7%. In my humble opinion, every White move is accurate (unless one considers 3. Bb5 to be more accurate than 3. Bc4). But why is the game accuracy for white is not 100%? What is the 0.3% that White l missed?

user15223: Assuming the engine performing the analysis is Stockfish (sounds reasonable, since the engine is presumably running on CPU) then it's probable the move it considers subpar is 4.c3. Left to its own devices Stockfish will occasionally play the Giuoco Piano, but it will usually play the Giuoco Pianissimo - i.e. 4.d3 instead of 4.c3.
Indeed, the cloud analysis by Stockfish 11 at lichess.org/analysis gives, at depth 45, +0.3 for 4.d3 compared to +0.2 for 4.c3. You can also check out the SF-Lc0 bookless bonus TCEC ran in 2019. There were ten Italian games, all played by Stockfish, and of these games 9/10 continued with 4.d3.

user-1: Well, we can trust the engine in some cases when they analyse, but in this case, we are looking at book moves. Since the Italian Game is perfectly reasonable, don't worry about the 0.3% off.

user23100: Probably, as you pointed out, Bb5 instead of Bc4.
The Ruy Lopez is considered very very marginally better/more critical than the Italian by humans, so it'd make sense that the engine used here'd agree.
Certainly, however, the Italian is fine, and the difference is very marginal.

16626: <p>Here is a short game played online at chess.com.</p>
<pre><code>[FEN &quot;&quot;]

1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. c3 d6 5. d4 Bb6 6. dxe5 
</code></pre>
<p>Black resigned for some unknown reason; maybe a mouse slip or he/she needs to leave. When the game was analysed at chess.com, the game accuracy of white is 99.7%. In my humble opinion, every White move is accurate (unless one considers 3. Bb5 to be more accurate than 3. Bc4). But why is the game accuracy for white is not 100%? What is the 0.3% that White l missed?</p>
 15223: <p>Assuming the engine performing the analysis is Stockfish (sounds reasonable, since the engine is presumably running on CPU) then it's probable the move it considers subpar is 4.c3. Left to its own devices Stockfish <em>will</em> occasionally play the Giuoco Piano, but it will usually play the Giuoco Pianissimo - i.e. 4.d3 instead of 4.c3.</p>
<p>Indeed, the cloud analysis by Stockfish 11 at lichess.org/analysis gives, at depth 45, +0.3 for 4.d3 compared to +0.2 for 4.c3. You can also check out the SF-Lc0 <a href="https://tcec-chess.com/#div=rapidfl&amp;game=17&amp;season=bonus" rel="nofollow noreferrer">bookless bonus</a> TCEC ran in 2019. There were ten Italian games, all played by Stockfish, and of these games 9/10 continued with 4.d3.</p>
 -1: <p>Well, we can trust the engine in some cases when they analyse, but in this case, we are looking at book moves. Since the Italian Game is perfectly reasonable, don't worry about the 0.3% off.</p>
 23100: <p>Probably, as you pointed out, Bb5 instead of Bc4.</p>
<p>The Ruy Lopez is considered very very marginally better/more critical than the Italian by humans, so it'd make sense that the engine used here'd agree.</p>
<p>Certainly, however, the Italian is fine, and the difference is very marginal.</p>
