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As fine-tuning becomes the dominant paradigm for improving large language models (LLMs), understanding what changes during this process is increasingly important. Traditional benchmarking often fails to explain textitwhy one model outperforms another. In this work, we use textbfmodel diffing, a mechanistic interpretability approach, to analyze the specific capability differences between textbfGemma-2-9b-it and a textbfSimPO-enhanced variant. Using textbfcrosscoders, we identify and categorize latent representations that differentiate the two models. We find that SimPO acquired latent concepts predominantly enhance safety mechanisms (+32.8\%), multilingual capabilities (+43.8\%), and instruction-following (+151.7\%), while its additional training also reduces emphasis on model self-reference (-44.1\%) and hallucination management (-68.5\%). Our analysis shows that model diffing can yield fine-grained insights beyond leaderboard metrics, attributing performance gaps to concrete mechanistic capabilities. This approach offers a transparent and targeted framework for comparing LLMs.